Projection objective for microlithography

ABSTRACT

A projection objective for use in microlithography, a microlithography projection exposure apparatus with a projection objective, a microlithographic manufacturing method for microstructured components, and a component manufactured under the manufacturing method are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims benefit under 35 USC120 to, international application PCT/EP2008/004081, filed May 21, 2008,which claims benefit of German Application No. 10 2007 024 685.6, filedMay 25, 2007 and U.S. Ser. No. 60/940,117, filed May 25, 2007.International application PCT/EP2008/004081 is hereby incorporated byreference in its entirety.

FIELD

The disclosure relates to a projection objective for use inmicrolithography, a microlithography projection exposure apparatus witha projection objective, a microlithographic manufacturing method formicrostructured components, and a component manufactured under themanufacturing method.

BACKGROUND

The performance of projection exposure apparatus for themicrolithographic production of semiconductor elements and other finelystructured components is largely determined by the imaging properties ofthe projection objectives. Examples for designs of projection objectivesof a projection exposure apparatus which project an image of a mask intoan exposure field can be found in WO 2004/019128 A2, US 2005/0190435 A1,WO 2006/133801 A1 and US 2007/0024960. These references relate primarilyto designs of projection objectives for immersion lithography, as thetechnique is called, wherein an immersion liquid is present between thelast optical element and the wafer which is located in the field planeof the exposure field. The subject of WO 2004/019128 A2, US 2005/0190435A1, WO 2006/133801 A1 and US 2007/0024960 in its entirety, including theclaims, is hereby incorporated by reference in the content of thepresent application. Furthermore, there are also designs of projectionobjectives of a projection exposure apparatus for applications inso-called EUV (extreme ultraviolet) lithography, which operate with anoperating wavelength of less than 100 nm and therefore generally cannotuse lenses as optical components, see US 2004/0051857 A1.

SUMMARY

The term “imaging properties” as commonly understood encompasses besidesthe point-to-point imaging properties also other kinds of imagingproperties such as for example the amount of stray light (hereinafterreferred to as the stray light component) contributed by the projectionobjective, because the contrast of the image is affected by it.

The stray light component of an objective has different reasons, whichare described in: Heinz Haferkorn, “Optik; Physikalisch-technischeGrundlagen and Anwendungen” (Optics, Physical and Technical Theory andApplications), Fourth Revised and Expanded Edition; Verlag WileY-VCH,Weinheim; pages 690-694. On the one hand, there is the kind of straylight which is caused by the scattering of light at inhomogeneitieswithin a transparent optical material, and on the other hand the kind ofstray light which is caused by the scattering of light at irregularitiesof the surfaces of the optical elements. Besides these two primarycauses of stray light, there are also secondary causes such as forexample double reflections, scattering which takes place at parts ofmounting devices, at borders of aperture stops and at walls, orscattering caused by undesirable dust particles. The foregoing secondarycauses of stray light are treated in the specialized literature alsounder the term “false light”. The secondary causes of stray light can bereduced considerably through a careful layout of the design, the mountsand aperture stops, as well as through increased cleanness, blackeningof the mount, and the development of effective so-called anti-reflexcoatings. In classic glass melts, a term which herein is meant to alsoinclude the quartz glass for the projection objectives used inmicrolithography, the inhomogeneities inside a transparent opticalmaterial can be small enclosed particles, minor variations of therefractive index, bubbles and striations. New kinds of opticalmaterials, in particular for projection objectives used in immersionlithography, are polycrystalline materials composed of a multitude ofindividual crystals of different sizes with hollow spaces of differentsizes lying between them, which will hereinafter also be referred to asbubbles (see WO 2006/061225 A1). The subject of WO 2006/061225 A1 in itsentirety, including the claims, is hereby incorporated by reference inthe content of the present patent application. In the polycrystallinematerials, not only the inhomogeneities in the form of bubbles are thereason for the stray light, but the base material itself in the form ofindividual small crystals causes stray light. This distinguishes the newmaterials from the classic materials, since the basic material of thelatter by itself generally causes no stray light except for smallvariations of the refractive index. This and the fact that significantlymore bubbles are present in the new materials than in the classicmaterials is the reason why optical elements made of the new kinds ofmaterials can generate much more stray light than would be generated byanalogous elements made of conventional material. In addition, many ofthe new materials consist of crystals that are birefringent, and a lightray traversing the material therefore sees many changes of therefractive index due to the different crystallographic orientations,whereby stray light can be produced again due to the refractive indexvariations themselves, as mentioned above. The many refractive indexvariations themselves, in turn, have the effect that the new kind ofmaterial itself hardly has a birefringent effect despite the fact thatit consists of many small crystals of birefringent material.

The elastic scattering of light of the wavelength λ at theinhomogeneities inside a transparent optical material can be treatedaccording to three different cases based on the diameter D of thescattering centers:

-   -   cases where D is small in comparison to λ are referred to as        Rayleigh scattering;    -   if D is about as large as λ, one speaks of Mie scattering, and    -   if D is significantly larger than λ, this is called geometric        scattering.

In each of these three cases different models are used in order todescribe the elastic scattering of light. In classic materials the Miescattering and the geometric scattering occur with predominance. In thenew kinds of materials, none of the aforementioned kinds of scatteringcan be disregarded because a sufficient number of bubbles between thecrystals can be very small and a sufficient number of individualcrystals may be very large as a source of scattering.

The elastic scattering of light of the wavelength λ which takes place atirregularities of surfaces is described through the theory ofdiffraction at gratings based on the assumption of a grating whoseheight equals the quadratic mean value of the height variation by whichthe irregularities deviate from the ideal surface and whose grid periodcorresponds to the mean local undulation wavelength of theirregularities. The quadratic mean value of the height variation of theirregularity from the ideal surface is also referred to as RMS value(root mean square value) of the surface roughness.

When characterizing the measurable qualities of a projection objective,an analysis as to which cause a measured stray light component of theprojection objective should be attributed to is a priori impossible.However, a measurable property through which stray light can becharacterized is based on different lateral penetrations into a shadowrange (see WO 2005/015313 and the references cited therein). Within thescope of conventional measurement methods, this property is tested byusing appropriate test masks which have dark areas of different lateraldiameters. In images of such masks which are produced by the projectionobjective, it is examined how large a portion of stray light is found inthe field of the projection objective at the center of the shadow rangeof the respective images of the individual dark areas. The diameters onthe image side for the images of the individual dark areas as measuredin the field plane of the projection objective are typically 10 μm, 30μm, 60 μm, 200 μm, 400 μm, 1 mm, and 2 mm. Such measurements areperformed at different field points in order to obtain the distributionof the stray light component over the exposure field of the projectionobjective.

Stray light which is still able to reach the center of a shadow range ofmore than 400 μm diameter has a range of more than 200 μm and is calledlong-range stray light, while stray light which reaches the center of ashadow range of less than 200 μm is referred to as short-range ormedium-range stray light. However, the transition between the terms isfluid so that an amount of 500 μm for the diameter of the shadow rangecan serve equally well as borderline between the terms of long-range orshort/medium-range stray light.

The stray light stemming from secondary causes for stray light isnormally not very localized or focused in the field plane, so that at acorresponding field point it normally extends uniformly over a lateralrange larger than for example 0.5 mm. This stray light belongsaccordingly to the long-range stray light and is thus representedequally in each measurement regardless of the diameters of the darkareas. This means that the long-range stray light is always present as abackground in a measurement of the short-range or medium range straylight.

To quantify the proportion of the stray light which is due to primarycauses through a measure that is not falsified by a stray lightcomponent that is due to secondary causes, the term “stray lightcomponent” as used herein is understood to mean only that part of thestray light which is obtained as the cumulative result of the individualmeasurements of the short-range portion up to a test diameter of 400 μm,where in each of the individual measurements of the short-range portionof the stray light the measurement result is reduced by the value of thestray light portion from the 1 mm measurement or an equivalent straylight measurement of the long-range portion. By setting this rule forthe stray light component within the bounds of this application, theshort-range portion of the stray light due to primary causes is thus setapart from the background of the long-range portion of the stray light.This clear delineation of the stray light portion due to primary causesis relevant because the long-range portion of the stray light due tosecondary causes contains the double reflections which, in turn, dependon the way in which the mask that is to be projected is illuminated.

It should also be noted here that as an alternative to the measurementof the stray light via sensors, the stray light can also be measuredthrough an exposure method for photoresists, the so-called Kirk test. Ina first step of this test, one determines the dose desired for thecomplete exposure of the photoresist, the so-called clearing dose D_(c),and in a second step one determines the dose D_(s) involved in anover-exposure of quadratic structures of different sizes, so that theirimage in the photoresist completely disappears.

The ratio between D_(c) and D_(s) now represents a measure for therelative stray light component of the square-shaped structure beingexamined.

Current projection objectives generally have a stray light component,according to the rule used herein, of about 1% in relation to the usefulportion of the light, wherein the stray light component varies by about0.2% over the image or over the exposure field. Starting from this, afurther reduction of the stray light component can be achieved through alarge development effort in regard to the material and the surfacefinish of mirrors and lenses. It should be noted, however, thatprojection objectives using the aforementioned new kinds of opticalmaterials will according to predictions have a larger stray lightcomponent and a higher variation of the stray light component.

In some embodiments, the disclosure ensures a good contrast over theimage or over the exposure field in projection objectives with at leastone optical element of polycrystalline material.

In certain embodiments, the disclosure provides a projection objectivethat includes a multitude of optical elements and has at least oneoptical element of polycrystalline material. The stray light componentof the projection objective, averaged over the scan direction, has avariation over the exposure field of less than 0.5%, such as less than0.2%, in relation to the useful light. Accordingly, the projectionobjective has a constant stray light component in the sense of thepresent application.

As used herein, a constant stray light component in the exposure field,averaged over the scan direction, means a stray light component forwhich the difference between the maximum value in the exposure field andthe minimum value in the exposure field in relation to the useful lightis less than 0.5% (e.g., less than 0.2%, less than 0.1%, less than0.05%), or for which the difference in relation to the maximum value inthe exposure field is less than 60% (e.g., less than 25%, less than12.5%, less than 6.75%).

The disclosure makes use of the observation that the variation of thestray light component of a projection objective over the exposure fieldoften causes greater problems for the manufacturers of semiconductorcomponents than a somewhat greater stray light component of theprojection objective would cause by itself, and that the variation ofthe stray light component over the exposure field in a projectionobjective with at least one optical element of polycrystalline materialexceeds the variation of the stray light component of many currentlyused projection objectives.

This result of an increased variation of the stray light component overthe exposure field in comparison to many currently used projectionobjectives is more pronounced if the last optical element consists ofpolycrystalline material. It can make sense especially in this case togenerate a constant stray light component of the projection objectiveover the exposure field in the sense of this application.

Setting an upper limit for the stray light component of a projectionobjective over the exposure field of 2% in relation to the useful light,takes into account that unrestrictedly large constant stray lightcomponents of a projection objective in the sense of this applicationare not compatible with the fabrication processes used by themanufacturers of semiconductor elements. Large stray light componentscan still lead to loss of contrast, and only small constant stray lightcomponents of a projection objective in the sense of this application,representing a low percentage of the useful light, are typicallyacceptable to the manufacturers of semiconductor elements.

Of comparable importance is the stray light component of a projectionobjective outside of the exposure field, because if it is too large itleads to undesirable exposures outside of the exposure field. Setting amaximum of 2% for the stray light component of a projection objectiveoutside of the exposure field can represent an acceptable upper limit.

The concept to introduce additional stray light in projection objectiveswith optical elements made of a fluoride, an oxide of group II, an oxideof group III, rare earth oxides, garnet or spinel leads to acompensation of the additional profile portion which the crystals andthe bubbles between the crystals contribute to the profile of the straylight component in the exposure field, so that the result is a constantstray light component of the projection objective in the sense of thisapplication over the entire exposure field.

The concept of introducing additional stray light in projectionobjectives with optical elements of a polycrystalline materialconsisting of many crystals that are birefringent leads to acompensation of the additional profile portion which the many refractiveindex fluctuations that occur as a result of the different orientationsof the crystals contribute to the profile of the stray light componentin the exposure field, so that the result is a constant stray lightcomponent of the projection objective in the sense of this applicationover the exposure field.

The concept of introducing additional stray light in projectionobjectives with at least one optical element of a polycrystallinematerial, so that the result is a constant stray light component in thesense of this application over the entire exposure field, where thepolycrystalline material exhibits a lesser degree of birefringence thaneach of the individual crystals, is especially important in projectionobjectives used for immersion lithography, because in these projectionobjectives a material that is nearly free of birefringence is used withpreference especially for the last optical element before the exposurefield.

The concept of introducing additional stray light in projectionobjectives with at least one optical element of a polycrystallinematerial, so that the result is a constant stray light component in thesense of this application over the entire exposure field, is ofparticular importance in cases where the optical element itself alreadyhas a stray light component with a profile variation of more than 0.1%over the exposure field, because in this case the individual opticalelement itself exhibits a variation of the stray light component overthe exposure field which equals about one-half the variation of thestray light component over the exposure field that is seen in currentlyused projection objectives.

In order to increase the resolution of future projection objectives usedfor immersion lithography, it may become desirable to further increasethe numerical aperture NA, i.e. the aperture angle. However, in order toaccomplish this, materials with a refractive index greater than 1.7 areneeded for the last optical element if the operating wavelength is forexample 193 nm. In this regard, the reader is referred to the discussionof the refractive index of the last lens element in WO 2006/133,801.With other operating wavelengths, too, such as for example 157 or 248nm, it is sensible to use a material with a high refractive index at therespective operating wavelength for the last lens element in projectionobjectives with a high aperture. The desired properties for the imagingperformance of such future systems, and likewise the desired propertiesfor the variation of the stray light component over the exposure field,will probably be higher than for present systems. The concept tointroduce additional stray light in projection objectives of this kind,so that the result is a constant stray light component in the sense ofthis application over the entire exposure field, takes this anticipateddevelopment into account, as the disclosure also provides the capabilityto meet increased future desired properties regarding the constancy ofthe stray light component over the exposure field.

Applying a finishing treatment to at least one surface of at least onefield-proximate optical element represents a simple and cost-effectiveway to introduce in a projection objective an additional stray lightcomponent, so that the result is a constant stray light component of theprojection objective in the sense of this application over the entireexposure field. The finishing treatment can also be applied to severalfield-proximate surfaces, so that the total additional stray lightcomponent comes out as the sum of the stray light contributed by theindividual surfaces. This distribution of the desired surface roughnessover several surfaces can be advantageous if it results for theindividual surface in a roughness value which can be realized simply byomitting the last polishing step on this surface or on parts of it.Field-proximate in this context means that surfaces close to anintermediate image rather than to the exposure field can also beselected for the finishing treatment. This is particularly advantageousif these surfaces are easier to work on in regard to their geometry, orif based on their optical sensitivity in regard to image errors, theyare easier to install or uninstall than the last optical elementimmediately before the exposure field. In particular a planar-parallelplate is favored as an optical element under this point of view, becausethe mechanical position tolerances that can be allowed for aplanar-parallel plate are much larger than for lenses or mirrors. Aplanar-parallel plate has the additional advantage that it can also bedesigned as an easily interchangeable element and thus offers thepossibility that this element can be exchanged or reworked or alteredaccording to customer specifications at a later time when the system isin operation.

Increasing the roughness of a field-proximate surface at the margin ofthe optically used area as compared to the center of the optically usedarea of a surface is the simplest way of producing in the exposure fieldan additional stray light component which has a profile over theexposure field and is stronger in the border area than in the centralarea of the exposure field, so that the overall result is a constantstray light component in the sense of this application over the entireexposure field for the projection objective as a whole. An additionalstray light component is thereby produced which complements theotherwise existing stray light component of the projection objective inan ideal way, so that the result is a constant stray light component ofthe projection objective in the sense of this application over theentire exposure field.

The difference of more than 0.5 nm between the respective RMS values forthe surface roughness at the margin of the optically used area of afield-proximate surface and the surface roughness at the center of theoptically used area corresponds to an additional stray light componentof about 0.02% in proportion to the useful light in the exposure fieldat an operating wavelength of e.g. 193 nm. The difference of 0.5 nmrepresents about the lower limit for a value for which it makes sense tocorrect the stray light component in the exposure field. The RMS valuelarger than 2 nm for the difference in the surface roughness from theborder to the center fills the task of correcting projection objectivescurrently used for microlithography with their variation of the straylight component over the exposure field of 0.2% relative to the usefullight at a wavelength of e.g. 193 by introducing an additional straylight component with a non-constant profile over the exposure field inaccordance with the disclosure, so that the result is a constant straylight component of the projection objective in the sense of thisapplication over the entire exposure field. Particularly in immersionobjectives used for immersion lithography, where the last lensimmediately before the field is strongly positive, an additionalvariation of the stray light component over the exposure area from theborder area to the central area occurs, and to compensate for thisvariation it makes sense to use a stronger differentiation of the RMSvalues of the surface roughness from the border to the central area.Additionally increased values for the difference in the RMS values ofthe surface roughness are involved if a strongly diffusing material isused for the last lens.

A surface roughness profile as a function of a lateral distance from thecenter, expressed through a root function of a general polynomialfunction in which the lateral distance represents the independentvariable offers the advantage of making it easier to program thepolishing machines, in particular the polishing robots, because a systemof functions is used which is indigenous or familiar to the machines.Disclosed herein are relatively simple and fast functions in thiscategory, which allow an increase of the RMS roughness value at themargin of a surface to be accomplished in the simplest and fastestpossible manner.

The local range of undulation wavelengths between 1 mm and 10 μm has theadvantage that it keeps the amount of so-called out-of-field stray lightsmall. The out-of-field stray light is stray light which gets outsidethe exposure field into areas where it can cause undesirable exposures.The local range of wavelengths between 1 mm and 10 mm has the advantagethat it not only has an effect on the stray light but also influencesthe image-forming wave front of a field point, so that it is possiblewith this local wavelength range to make a simultaneous correction ofthe wave front of an arbitrary field point.

A field aperture stop can help prevent the additional stray light, whichwas introduced to achieve the result of a constant stray light componentof the projection objective in the sense of this application over theentire exposure field, from getting into areas outside of the exposurefield and leading to undesirable exposures of those areas.

The dimensional allowance between the field aperture stop and theoptically used area in the plane of the field aperture stop representsan advantageous compromise between an overly tight allowance which leadsto a high cost due to the high precision desired in the manufacturingprocess and an overly large allowance which leads to too muchundesirable stray light outside of the exposure field.

An upper side, i.e. the object-facing surface of the last opticalelement before the field plane, as seen in the direction of the lightrays from the mask plane to the field plane, is advantageously suitedfor introducing the stray light by surface roughness, as this surface ison the one hand located so close to the exposure field that by a profileof the surface roughness over the upper side a profile of the straylight component in the exposure field can be produced, and that on theother hand the sub-apertures of the individual field points on the upperside are still wide enough that small irregularities in the finish ofthe upper side have no effect on the image of the respective fieldpoints. Particularly in projection objectives used for immersionlithography, it is especially advantageous to finish the upper side ofthe last optical element because, due to the small difference in therefractive indices of the lens and the immersion liquid, finishing orreworking of the underside would involve very large values for thesurface roughness which are difficult to achieve in practice.

In projection objectives used for immersion lithography, the designspace between the last optical element and the wafer is too narrow toallow the use of mechanical aperture stops. The concept of maskingtherefore represents the best possible way of realizing a field aperturestop in immersion systems, which prevents the additional stray light,which was introduced to achieve the result of a constant stray lightcomponent of the projection objective in the sense of this applicationover the entire exposure field, from getting into areas outside of theexposure field and leading to undesirable exposures of those areas.

The masking is realized cost-effectively by a.

The dimensional allowance between the masking and the optically usedarea in the plane of the field aperture stop represents an advantageouscompromise between an overly tight dimensional allowance which leads toa high cost due to the high manufacturing precision desired inparticular for coating tools and an overly large allowance which leadsto too much undesirable stray light outside of the exposure field.

Particularly in immersion objectives for use in immersion lithography,where the refractive power of the last lens immediately before the fieldis strongly positive, this strongly curved lens alone has the effectthat the path lengths traveled by the light rays through the materialdiffer by a few percent for rays traversing the border area incomparison to rays passing through the central area, which results in anadditional variation of the stray light component over the exposurefield. This effect is further increased if strongly diffusing materialis used for the last lens. The concept of introducing additional straylight, so that the overall result is a constant stray light component ofthe projection objective in the sense of this application over theentire exposure field, is therefore advantageous to reduce the variationof the stray light component over the exposure field in projectionobjectives, which have a last lens of polycrystalline material withpositive refractive power.

Using a planar-parallel plate as the last optical element has theadvantage that the planar-parallel plate allows for large mechanicalposition tolerances in comparison to lenses or mirrors and that it isthus optically insensitive. This kind of optical element is thereforeadvantageous in regard to reworking operations, as it can be uninstalledfrom and reinstalled in the projection objective without major problems.A refinishing operation at the customer's location is thereby also madepossible, so that an adjustment of the stray light profile according toa customer's wish becomes feasible. This customer request could beconnected for example with a specific illumination of the mask.

A surface roughness of a mirror surface has an approximately 16 timesstronger effect than an equivalent surface roughness of a lens in airwith a refractive index of about 1.5. It is insofar advantageous, iflarge variations of the stray light component over the exposure fieldhave to be corrected, to use for this purpose a mirror surface so thatthe overall result is a constant stray light component of the projectionobjective in the sense of this application over the entire exposurefield.

It is a further object of the disclosure to reduce the variation of thestray light component over the image or over the exposure field.

The disclosure makes use of the observation that the variation of thestray light component over the exposure field causes greater problems tothe manufacturers of semiconductor components than the stray lightcomponent itself.

This task can be solved by a projection objective with the features inwhich an additional stray light component is introduced with anon-constant profile over the exposure field, or that a mechanism isprovided in the projection objective for introducing into the exposurefield in the field plane an additional stray light component with anon-constant profile over the exposure field. The property of anadditional stray light component as having a non-constant profile overthe exposure field in this context is understood to mean a profile ofthe additional stray light component wherein for at least two arbitraryfield points within the exposure field there is a difference of ≧0.02%in the additional stray light component in relation to the useful lightportion. Thus, a projection objective is made available for use inmicrolithography, serving to project an image of a mask plane into afield plane and having an exposure field in the field plane, which ischaracterized by the fact that besides the existing stray lightcomponent of the projection objective an additional stray lightcomponent is introduced with a non-constant profile over the exposurefield, and/or that the projection objective includes a mechanism wherebybesides the existing stray light component of the projection objectivean additional stray light component with a non-constant profile over theexposure field is introduced into the exposure field, so that thevariation of the stray light component over the exposure field isreduced.

It was further recognized that it makes sense for any optical body ifthe stray light component in the border area of the exposure field isincreased in comparison to the central area of the exposure field inorder to equalize over the exposure field the profile of the stray lightcomponent which stems from a homogeneous light flow of the useful lighteven if the latter takes place only in part of the optical body. Thisentails the precondition that the optical body consists of a homogeneousmaterial and has homogeneously finished surfaces, as for example a lensor a plurality of lenses of a projection objective. Particularly inimmersion objectives for use in immersion lithography, where therefractive power of the last lens immediately before the field isstrongly positive, this strongly curved lens alone has the effect thatthe path lengths traveled by the light rays through the material differby a few percent for rays traversing the border area in comparison torays passing through the central area, which results in an additionalvariation of the stray light component, with an increased proportion inthe central area and a lower proportion in the border area of theexposure field. This effect is further increased if strongly diffusingmaterial is used.

The finishing treatment of at least one surface of at least one opticalelement close to the field (also referred to herein as a field-proximateelement) represents a simple and cost-effective way to introduce in aprojection objective an additional stray light component with anon-constant profile over the exposure field. The finishing treatmentcan also be applied to several field-proximate surfaces, so that thetotal additional stray light component comes out as the sum of the straylight contributed by the individual surfaces. This distribution of theadditional surface roughness over several surfaces can be advantageousif it results for the individual surface in a roughness value which canbe realized simply by omitting the last polishing step on this surfaceor on parts of it. Close to a field (or field-proximate) means in thiscontext that surfaces close to an intermediate image instead of close tothe exposure field can also be selected for the finishing treatment.This is particularly advantageous if these surfaces are easier to workon in regard to their geometry, or if based on their optical sensitivityin regard to image errors, they are easier to install or uninstall thanthe last optical element immediately before the exposure field. Inparticular a planar-parallel plate is favored as an optical elementunder this point of view, because the mechanical position tolerancesthat can be allowed for a planar-parallel plate are much larger than forlenses or mirrors. A planar-parallel plate has the additional advantagethat it can also be designed as an easily interchangeable element andthus offers the possibility that this element can be exchanged orreworked or altered according to customer specifications at a later timewhen the system is in operation.

Increasing the surface roughness at the margin of the optically usedarea as compared to the center of the optically used area of a surfacenear a field (also referred to herein as a field-proximate surface) isthe simplest way of producing in the exposure field an additional straylight component which has a profile over the exposure field and isstronger in the border area than in the central area of the exposurefield. An additional stray light component is thereby produced whichcomplements the otherwise existing stray light component of theprojection objective in an ideal way.

The difference of more than 0.5 nm between the respective RMS values forthe surface roughness at the margin of the optically used area of afield-proximate surface and the surface roughness at the center of theoptically used area corresponds to an additional stray light componentof about 0.02% in proportion to the useful light in the exposure fieldat an operating wavelength of e.g. 193 nm. The difference of 0.5 nmrepresents about the lower limit for a value for which it makes sense tocorrect the stray light component in the exposure field. The RMS valuelarger than 2 nm for the difference in the surface roughness from theborder to the center fills the task of correcting projection objectivescurrently used for microlithography with their variation of the straylight component over the exposure field of 0.2% relative to the usefullight at a wavelength of e.g. 193 by introducing an additional straylight component with a non-constant profile over the exposure field inaccordance with the disclosure.

Particularly in immersion objectives for use in immersion lithography,where the refractive power of the last lens immediately ahead of thefield is strongly positive, a stronger variation of the stray lightcomponent over the exposure field from the border area to the centralarea occurs, as mentioned previously, where it makes sense to compensatefor the variation by using larger values for the difference in RMSsurface roughness from the border to the center. Additionally increasedvalues for the difference in the RMS surface roughness are needed ifstrongly diffusive material is used for a last lens in this kind ofarrangement.

The profile of the surface roughness as a function of a lateral distancefrom the center according to a function represented by the root of ageneral polynomial function in which the lateral distance is theindependent variable offers the advantage of making it easier to programthe polishing machines, in particular the polishing robots, because asystem of functions is used which is indigenous or familiar to themachines. Relatively simple and fast functions in this category aredisclosed, which allow an increase of the RMS roughness value at theborder of a surface to be accomplished in the simplest and fastestpossible manner.

The range of wavelengths of the local undulation between 1 mm and 10 μmhas the advantage that it keeps the amount of so-called out-of-fieldstray light small. The out-of-field stray light is stray light whichgets outside the exposure field into areas where it may causeundesirable exposure to light. The local range of undulation wavelengthsbetween 1 mm and 10 mm has the advantage that it not only has an effecton the stray light but also influences the image-forming wave front of afield point, so that it is possible with this local wavelength range tomake a simultaneous correction of the wave front of an arbitrary fieldpoint. As mentioned above, the local wave length range of a surfaceroughness or irregularity is understood within the bounds of thisapplication to mean the range of the lateral grid periods of theirregularities along the surface of an optical element.

A field aperture stop can help prevent the additionally introduced straylight from getting into areas outside of the exposure field and leadingto undesirable exposures of those areas.

The dimensional allowance between the field aperture stop and theoptically used area in the plane of the field aperture stop representsan advantageous compromise between an overly tight allowance which leadsto a high cost due to the high precision desired in the manufacturingprocess and an overly large allowance which leads to too muchundesirable stray light outside of the exposure field.

An upper side, i.e. the object-facing surface of the last opticalelement, is advantageously suited for introducing the stray light bysurface roughness, because this surface is on the one hand located soclose to the exposure field that by a profile of the surface roughnessover the upper side a profile of the stray light component in theexposure field can be produced, and because on the other hand thesub-apertures of the individual field points on the upper side are stillwide enough that small irregularities in the finish of the upper sidehave no effect on the image of the respective field point. Particularlyin projection objectives used for immersion lithography, the finish ofthe upper side of the last optical element is especially importantbecause, due to the small difference in the refractive indices of thelens and the immersion liquid, finishing or reworking of the undersidewould lead to large values for the surface roughness, which would have anegative effect on the imaging properties of the projection objective oron the dynamics of the immersion liquid during the scanning process.

In projection objectives used for immersion lithography, the designspace between the last optical element and the wafer is too narrow toallow the use of mechanical aperture stops. The concept of masking istherefore almost the only possible way in immersion systems to realize afield aperture stop which prevents the additionally introduced straylight from getting into areas outside of the exposure field and leadingto undesirable exposures of those areas.

The masking is realized cost-effectively by a coating

The dimensional allowance between the masking and the optically usedarea in the plane of the field aperture stop represents an advantageouscompromise between an overly tight dimensional allowance which leads toa high cost due to the high manufacturing precision desired inparticular for coating tools and an overly large allowance which leadsto too much undesirable stray light outside of the exposure field.

The concept of introducing additional stray light is especiallyadvantageous in projection objectives with optical elements ofpolycrystalline material as the polycrystalline material in theseprojection objectives causes a stronger variation of the stray lightcomponent over the field than would be the case in currently usedprojection objectives.

The concept of introducing additional stray light in projectionobjectives with optical elements made of a fluoride, an oxide of groupII, an oxide of group III, rare earth oxides, garnet or spinel leads toa compensation of the additional profile portion which the crystals andthe bubbles between the crystals contribute to the profile of the straylight component in the exposure field.

The concept of introducing additional stray light in projectionobjectives with optical elements of a polycrystalline materialconsisting of many crystals that are birefringent leads to acompensation of the additional profile portion which the many refractiveindex fluctuations that occur as a result of the different orientationsof the crystals contribute to the profile of the stray light componentin the exposure field.

The concept of introducing additional stray light in projectionobjectives with at least one optical element of a polycrystallinematerial which exhibits a lesser degree of birefringence than each ofthe individual crystals is especially important for projectionobjectives used for immersion lithography, because in these projectionobjectives a material that is nearly free of birefringence is used withpreference especially for the last optical element before the exposurefield.

The concept of introducing additional stray light in projectionobjectives with at least one optical element of a polycrystallinematerial represents a sensible approach in particular if the opticalelement itself already has a stray light component with a profilevariation of more than 0.1% over the exposure field because in this casethe individual optical element itself exhibits a variation of the straylight component over the exposure field which equals about one-half thevariation of the stray light component over the exposure field that isseen in currently used projection objectives.

In particular a last optical element of polycrystalline material locatedbefore the field plane, in reference to the direction of a light rayfrom the mask plane to the field plane leads to a stronger variation ofthe stray light component of a projection objective over the exposurefield, which needs to be compensated in accordance with the disclosure,because downstream of such a field-proximate optical element there is nofurther possibility to place aperture stops immediately ahead of thefield plane with the exposure field in order to prevent the stray lightgenerated by this element from reaching the exposure field.

In order to increase the resolution of future projection objectives usedfor immersion lithography, it will probably be desirable to furtherincrease the numerical aperture NA, i.e. the aperture angle. However, inorder to accomplish this, materials with a refractive index greater than1.7 are needed for the last optical element if the operating wavelengthis for example 193 nm. In this regard, the reader is referred to thediscussion of the refractive index of the last lens element in WO2006/133,801 A1. With other operating wavelengths, too, such as forexample 157 or 248 nm, it is sensible to use a material with a highrefractive index at the respective operating wavelength for the lastlens element in projection objectives with a high aperture. The desiredproperties for the imaging performance of such future systems, andlikewise the desired properties for the variation of the stray lightcomponent over the exposure field, will probably be higher than forpresent systems. The concept to introduce additional stray light inprojection objectives of this kind takes this anticipated developmentinto account, as the disclosure also provides the capability to meetincreased future desired properties for the variation of the stray lightcomponent over the exposure field.

Particularly in immersion objectives for use in immersion lithography,where the refractive power of the last lens immediately before the fieldis strongly positive, this strongly curved lens alone has the effectthat the path lengths traveled by the light rays through the materialdiffer by a few percent for rays traversing the border area incomparison to rays passing through the central area, which results in anadditional variation of the stray light component. This effect isfurther increased if strongly diffusing material is used for the lastlens. The concept of introducing additional stray light in suchprojection objectives is thus helpful in reducing the variation of thestray light component over the exposure field in projection objectiveswith a last lens of positive refractive power.

Using a planar-parallel plate as the last optical element has theadvantage that the planar-parallel plate allows for large mechanicalposition tolerances in comparison to lenses or mirrors and that it isthus optically insensitive. This kind of optical element is thereforeadvantageous in regard to reworking operations, as it can be uninstalledfrom and reinstalled in the projection objective or exchanged foranother planar-parallel plate without major problems. A refinishingoperation at the customer's location is thereby also made possible, sothat an adjustment of the stray light profile according to a customer'swish becomes feasible. This customer request could be connected forexample with a specific illumination of the mask.

A surface roughness of a mirror surface has an approximately 16 timesstronger effect than an equivalent surface roughness of a lens in airwith a refractive index of about 1.5. It is insofar advantageous, iflarge variations of the stray light component over the exposure fieldhave to be corrected, to use for this purpose a mirror surface.

A further object of the disclosure is to provide a method of reducingthe variation of the stray light component of a projection objective inthe exposure field.

This task can be solved by a method in which additional stray light witha non-constant profile over the exposure field is introduced by anadvance adaptation or an alteration of the surface roughness of at leastone field-proximate surface.

It was recognized in the disclosure that a method in which the surfaceroughness of at least one field-proximate surface is adapted in advanceaccording to a specified profile over the surface or altered in a waythat is targeted to achieve the specified profile represents a suitableway to produce an additional stray light component which results in anoverall reduction of the variation of the stray light component over theexposure field.

A method in which a simulation or a measurement is used to determine thestray light component that is to be expected or is present within theexposure field of the entire projection objective, offers thepossibility to determine the desired surface roughness profile over theat least one field-proximate surface in a way that is very specificallytargeted and to realize the desired profile in an equallytarget-oriented way by pre-adapting or altering the surface roughness.

A method can offer the advantage of taking the measurements on anotherprojection objective of the same design, for example on a prototype,instead of measuring the projection objective itself, and to transferthe results to the projection objective for making the correction. Thissaves expensive and risky corrective steps in the manufacturing process,where the already completed projection objective has to be disassembledagain, i.e. the steps of measuring the projection objective,uninstalling the surface that needs to be changed, reworking thesurface, and reinstalling the reworked surface. By transferring themeasurement results for example from a prototype, the desired surfaceroughness can be preset or adapted in advance already during theproduction of the optical elements of the projection objective.

A method can have the advantage that measurements which were alreadymade in the production of the individual optical components can be usedfor determining the stray light component to be expected for the entireprojection objective, so that the surface roughness of the at least onefield-proximate surface can be adapted in advance already in theproduction of the respective optical element, without having todisassemble the projection objective again at a later stage of theproduction process.

If no measurements are performed on the optical components in regard tothe stray light component to be expected, the method can offer theadvantageous possibility to take such measurements on the blanks of thelenses. The blanks can all be measured with one and the same measurementsetup, while lenses may in some cases involve different measurementsetups, depending on the geometry of the lens. Performing a measurementregarding the existing stray light component on the blanks offersinsofar a significant cost advantage over a measurement of the straylight component that is performed in any of the subsequent productionsteps.

A method can be very cost-effective, because for the determination ofthe stray light component of the projection objective only at least onelens is measured or simulated for the advance adaptation or subsequentalteration of the surface roughness of the at least one field-proximateoptical surface, rather than making measurements on an entire projectionobjective which would involve a more extensive measuring apparatus. Thisis of particular interest for projection objectives used for immersionlithography with a last lens of polycrystalline material, where thisindividual lens alone already contributes a large portion of the straylight component of the projection objective and where the attention isfocused on correcting this particular contribution to the stray lightcomponent in accordance with the disclosure.

In comparison to certain methods it is advantageous to use some methodsbecause only a single measurement needs to be made, e.g. on a lensprototype consisting, e.g., of polycrystalline material, in order tomake the correction in all projection objectives that contain such alens without the need to measure each of the individual lenses byitself. It is also possible with certain methods to perform a randomsample examination within the scope of a quality assurance program,wherein for the determination of the desired surface roughness of the atleast one field-proximate surface a second lens is measured which is ofidentical design as the first lens of the projection objective and theresults of the measurements from the second lens are applied to thefirst lens.

A method is disclosed in which instead of measuring a lens, themeasurements are made on the blank from which the lens will be made, inorder to obtain from the measurement results of the blank the data forthe advance adaptation or subsequent alteration of the surface roughnessof the at least one field-proximate surface, so that one obtains as aresult the additional stray light component with the non-constantprofile over the exposure field of the projection objective. This methodis simple and cost-effective because a suitable measurement setup for ablank can be realized in a simpler and more cost-effective way than acorresponding measurement setup for a completed lens or an entireobjective.

It is a further object of this disclosure to provide a method ofintroducing additional stray light by a preemptive adaptation orsubsequent alteration of the surface roughness of at least onefield-proximate surface, so that as a result the stray light componentof the projection objective, averaged over the scan direction, variesover the exposure field by less than 0.5%, in particular less than 0.2%,in relation to the useful light, and accordingly a constant stray lightcomponent of the projection objective in the sense of this applicationis achieved.

It was recognized in the disclosure that a method in which the surfaceroughness of at least one field-proximate surface is adapted in advanceaccording to a specified profile over the surface or changed in a waythat is targeted to achieve the specified profile represents a suitableway to produce an additional stray light component which results in anoverall reduction of the variation of the stray light component over theexposure field, so that a constant stray light component of theprojection objective in the sense of this application is achieved.

A method is disclosed in which a simulation or a measurement is used todetermine the stray light component that is to be expected or is presentwithin the exposure field of the entire projection objective, offers thepossibility to determine the desired surface roughness profile over theat least one field-proximate surface in a way that is very specificallytargeted and to realize the desired profile in an equallytarget-oriented way by adapting it in advance or changing it, so that aconstant stray light component of the projection objective in the senseof this application is achieved.

A method is disclosed that offers the advantage of taking themeasurements on another projection objective of the same design, forexample on a prototype, instead of measuring the projection objectiveitself, and to transfer the results to the projection objective formaking the correction. This saves expensive and risky corrective stepsin the manufacturing process, where the already completed projectionobjective has to be disassembled again, i.e. the steps of measuring theprojection objective, uninstalling the surface that needs to be changed,reworking the surface, and reinstalling the reworked surface. Bytransferring the measurement results for example from a prototype, thedesired surface roughness can be preset or adapted in advance alreadyduring the production of the optical elements of the projectionobjective, so that a constant stray light component of the projectionobjective in the sense of this application is achieved.

A method is disclosed that is very cost-effective, because for thedetermination of the stray light component of the projection objectiveonly at least one lens is measured or simulated for the advanceadaptation or the alteration of the surface roughness of the at leastone field-proximate optical surface in order to achieve a constant straylight component of the projection objective in the sense of thisapplication, rather than making measurements on an entire projectionobjective which would involve a more extensive measuring apparatus. Thisis of particular interest for projection objectives used for immersionlithography with a last lens of polycrystalline material, where thisindividual lens alone already contributes a large portion of the straylight component of the projection objective and where the attention isfocused on correcting this particular contribution to the stray lightcomponent in accordance with the disclosure in order to achieve aconstant stray light component of the projection objective in the senseof this application over the entire exposure field.

In comparison to the method according to some methods it is advantageousto use certain methods because only a single measurement needs to bemade, e.g. on a lens prototype consisting, e.g., of polycrystallinematerial, in order to make the correction in all projection objectivesthat contain such a lens so that a constant stray light component of theprojection objective in the sense of this application is achievedwithout the need to measure each of the individual lenses by itself. Itis also possible with certain methods to perform a random sampleexamination within the scope of a quality assurance program, wherein forthe determination of the desired surface roughness of the at least onefield-proximate surface a second lens is measured which is of identicaldesign as the first lens of the projection objective and the results ofthe measurements taken from the second lens are applied to the firstlens, so that a constant stray light component of the projectionobjective in the sense of this application is achieved.

A method is disclosed in which instead of measuring a lens, themeasurements are made on the blank from which the lens will be made, inorder to obtain from the measurement results of the blank the data forthe advance adaptation or the alteration of the surface roughness of theat least one field-proximate surface, so that one obtains as a resultthe additional stray light component with the non-constant profile overthe exposure field of the projection objective. This method is simpleand cost-effective because a suitable measurement setup for a blank canbe realized in a simpler and more cost-effective way than acorresponding measurement setup for a completed lens or an entireobjective.

It is a further object of the disclosure to provide a projectionexposure apparatus with a projection objective, also to provide amicrolithographic manufacturing method which can be performed with theprojection exposure apparatus, and further to describe a component whichcan be manufactured with the apparatus and method.

The disclosure provides a projection exposure apparatus, a manufacturingmethod, and a component.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments of the disclosure are hereinafter presented inmore detail with references to the drawing, wherein

FIG. 1 is a schematic representation of an exposure field in the fieldplane of a projection objective for microlithography applications whichis used as a scanner, including the distribution of the useful lightrelative to two orthogonal axes (X- and Y-axis);

FIG. 2 is a schematic representation of an exposure field in the fieldplane of a projection objective for microlithography applications whichis used as a scanner and has a so-called off-axis field of rectangularshape;

FIG. 3 is a schematic representation of an exposure field in the fieldplane of a projection objective for microlithography applications whichis used as a scanner and has a so-called ring field;

FIG. 4 is a schematic representation of an exposure field in the fieldplane of a projection objective for microlithography applications whichis used as a stepper and has a square field;

FIG. 5 shows a schematically simplified sectional view of a projectionobjective and a substitute model for the projection objective in theform of a homogeneous glass cylinder serving to explain the resultantnatural stray light distribution;

FIG. 6 shows a schematic representation of an image-forming light raypattern of a projection objective according to geometric optics toillustrate the concepts of field and pupil;

FIG. 7 represents a graph of the profile of the stray light component inpercent in relation to the useful light of a projection objective formicrolithography applications, averaged over the scan direction Y, alongthe field in the X-direction;

FIG. 8 schematically represents the optical components of a projectionexposure apparatus for immersion lithography;

FIG. 9 represents a plan view of a polycrystalline material with itmicroscopic structures;

FIG. 10 represents a graph of a model-dependent stray light component,expressed in percent relative to the useful light, of a polycrystallinematerial as a function of the average crystal size;

FIG. 11 represents a graph of a model-dependent stray light component,expressed in percent relative to the useful light, of a polycrystallinematerial as a function of the average bubble size;

FIG. 12 represents a sketch to illustrate principal concepts in a lensand graphs to explain, respectively, the scattering at inhomogeneitiesin a polycrystalline material of a last lens and the concept of adaptingthe surface roughness of a last lens as well as the resultantdistribution of stray light over the field;

FIG. 13 represent a graph of a corrected profile of the stray lightcomponent, expressed in percent relative to the useful light, of aprojection objective for microlithography applications, averaged overthe scan direction Y along the field in the X-direction;

FIG. 14 a-b represents a sectional view in the Y-Z plane of the opticalcomponents of a so-called two-mirror design of a projection objectivefor immersion lithography with a numerical aperture larger than 1;

FIG. 15 represents a sectional view in the Y-Z plane of the opticalcomponents of a so-called four-mirror design of a projection objectivefor immersion lithography with a numerical aperture of 1.2;

FIG. 16 represents a sectional view in the Y-Z plane of the opticalcomponents of a so-called RCR design of a projection objective forimmersion lithography with a numerical aperture of 1.25;

FIG. 17 represents a sectional view in the Y-Z plane of the opticalcomponents of a further two-mirror design of a projection objective forimmersion lithography with a numerical aperture of 1.75;

FIG. 18 schematically illustrates the last lens element before the fieldplane of the two-mirror design of FIG. 17;

FIG. 19 represents a sectional view of the optical components of aso-called six-mirror design of a projection objective for EUVlithography;

FIG. 20 represents a graph of a possible distribution of the surfaceroughness over the optically used area of a surface of a field-proximateoptical element;

FIG. 21 represents a flowchart diagram of several possible process stepsfor producing in a projection objective a corrected stray lightcomponent;

FIG. 22 represents a flowchart diagram for a method of producingmicrostructured semiconductor elements by a projection exposureapparatus with a projection objective in accordance with the presentpatent application.

DETAILED DESCRIPTION

FIG. 1 shows the exposure field 15 in the field plane of a projectionobjective for microlithography applications which is used as scanner,including the distribution of the useful light along the X- and Y-axes.In FIG. 1, the field plane in which the exposure field 15 is located isseen in plan view, meaning that the plane of the paper coincides withthe field plane. Further in FIG. 1, a coordinate system is defined inthe field plane in accordance with the rule that for so-called scannersthe scan direction should be oriented in the Y-direction. In so-calledscanners, the mask structure of a microstructured component is nottransferred in its entirety in one exposure step by the projectionobjective onto a so-called wafer, because the image of the entire maskstructure is too large for the maximum image field 1 of a projectionobjective. Instead, the mask structure is gradually moved through theobject- or mask plane of the projection objective in a scanning process,while the wafer is moved at the same time in a synchronized movementthrough the image- or field plane. In conventional rotationallysymmetric projection systems, which have refractive elementsexclusively, the maximum image field 1 in the field plane is a circlewhose center is defined by the optical axis 3 of the projection system.By field aperture stops which are located in the illumination system,the so-called REMA (reticle-masking) blades, the maximum image field 1is trimmed back to the rectangular exposure field 15 whose center isdefined by the optical axis 3 of the objective. The REMA blades have theadditional function at the beginning and end of a scanning process,respectively, to retract and deploy themselves over the exposure field15. The center of the exposure field 15 is formed by a central area 5which is shaded in FIG. 1. The border areas (also referred to herein asmarginal areas) 7 and 9 of the exposure field, which are likewise shadedin FIG. 1, are those border areas 7 and 9 of the rectangular exposurefield which form the left and right margins of the exposure field in thedirection perpendicular to the scan direction. In the scan direction,the front edge 11 and the rear edge 13 of the exposure field 15 are thelines between which the exposure field 15 is located and between whichthe light projected by the objective has an intensity larger than zero.The respective intensity distribution profiles of the useful light inthe scan direction and perpendicular to the scan direction are differentfrom each other, as shown in the two diagrams in FIG. 1. In the scandirection, the intensity distribution profile of the useful light isadjusted so that it takes on a value of zero at the front edge 11 andthe rear edge 13 and has its maximum within the central area 5. Theexact intensity distribution between these two points is selected sothat in the scanning process every partial area of a microstructuredcomponent receives a nearly equal portion of the light. This would beimpossible to achieve with a so-called top hat profile, i.e. arectangular distribution profile in the scan direction, because a pulsedlaser is typically used as a light source and it could not be ruled outin this case that one partial area of the microstructured componentwould receive light from one more laser pulse than another partial area,with 5 to 7 laser pulses per partial area or per exposure field beingtypical. With an intensity distribution in the scan direction whichcontinuously increases towards the central area 5 from a value of zeroat the front edge 11 and at the rear edge 13, such intensity effects onthe microstructured components are suppressed.

In contrast, the intensity distribution perpendicular to the scandirection is a so-called top hat distribution or rectangulardistribution over the exposure field 15, with the same intensity valuefor the central area 5, the border areas 7 and 9 and all field pointslying in between along a line that is perpendicular to the scandirection. Insofar, the shape of this intensity distribution also doesnot change if it is averaged over the scan direction. This intensitydistribution, averaged over the scan direction and expressed in percentrelative to the useful light is represented by the diagram in the bottompart of FIG. 1. This averaged intensity distribution has the same valueof 100% relative to the useful light for the central area 5 as for theborder area 7.

The stray light component defined according to the measuring rule statedabove is understood herein as a stray light component that is averagedover the scan direction and expressed as a relative amount in proportionto the useful light or, in other words, as a relative amount inproportion to the 100% value of the intensity distribution in the scandirection as illustrated in FIG. 1.

The exposure field 15 of a scanner typically measures 20 to 30 mmperpendicular to the scan direction and 5 to 10 mm in the scandirection. Together with these dimensions, the central area 5 of theexposure field 15 should not exceed a diameter of 4 mm, and the borderareas 7 and 9 of the exposure field 15 should not exceed a width of 2 mmperpendicular to the scan direction, as these areas should only occupysmall surface portions immediately at the center and at the border ofthe exposure field 15 without spreading out over major portions of theexposure field 15.

FIG. 2 shows the exposure field 45 in the field plane of a projectionobjective for microlithography applications which is used as a scannerand has a so-called off-axis field 45 of rectangular shape as exposurefield 45. The elements in FIG. 2 which are analogous to those in FIG. 1have the same reference numerals raised by 30. Such rectangular off-axisfields 45 as exposure fields 45 of a projection objective are typical inprojection objectives which have at least one catadioptric partialobjective. The attribute “catadioptric” means here that besidesrefractive elements such as for example lenses, there are alsoreflective elements such as for example mirrors being used as elementswhich contribute to the formation of the image and thus carry refractivepower. Due to the folded ray path of these systems, the exposure field45 is offset relative to the optical axis 33 and the maximum image field31 of these systems. When referring to the optical axis 33 and themaximum image field 31 in this context, this does not imply that theoptical axis 33 as well as the entire maximum image field 31 can becovered in the projected image of these catadioptric projectionobjectives. It only indicates that many of these catadioptric projectionobjectives can still be described in terms of rotational symmetry inregard to their design, even though the ray propagation pattern used inthe completed objective is not folded with rotational symmetry relativeto the optical axis 31 and the physical shapes of some of the opticalelements are no longer rotationally symmetric relative to the opticalaxis 31. Examples for the design of a catadioptric projection objectivewith a rectangular off-axis field 45 as exposure field 45 are presentedin US 2005/0190435 A1, WO 2004/019128 A2 and WO 2006/133801 A1, as wellas in FIGS. 14, 16 and 17 of the present patent application. What hasbeen the above in the context of FIG. 1 about the intensity distributionin the scan direction and perpendicular to it is also directlyapplicable to the rectangular off-axis field 45 and therefore needs nofurther explanation. Rectangular off-axis fields 45 of catadioptricprojection objectives have about the same size as exposure fields 15 ofpurely refractive projection objectives. Catadioptric projectionobjectives are used primarily for immersion lithography because evenwith the large numerical aperture values (NA) of more than 1 of animmersion objective, catadioptric projection objectives allow the lens-and mirror diameters to be kept relatively small in comparison to apurely refractive design.

FIG. 3 shows the exposure field 65 in the field plane of a projectionobjective for microlithography applications which is used as a scannerand has a so-called ring field 65 as exposure field 65. The elements inFIG. 3 which are analogous to those in FIG. 1 have the same referencenumerals raised by 50. Such ring fields 65 are typical for catadioptricobjectives of a design that does not allow for a folded light ray paththat would lead to a rectangular field. What has been the above in thecontext of FIG. 1 about the intensity distribution in the scan directionand perpendicular to it is also directly applicable to the ring field 65and therefore needs no further explanation. The intensity distributionin the scan direction can differ from the intensity distribution shownin FIG. 1 insofar as with different heights in the X-direction theresultant distribution is not the same for all intensity distributionsin the scan direction. However, this is of no consequence, and it wouldalso be of no consequence if it occurred in a system with a rectangularfield 15, 45, as all scanner systems are always designed so thatregardless of the shape of the intensity distribution along the scandirection, one always obtains an intensity distribution perpendicular tothe scan direction which, when averaged over the scan direction,conforms to a top-hat profile or rectangular profile of the typeillustrated in the lower part of FIG. 1. Ring fields 65 of catadioptricprojection objectives have about the same dimension perpendicular to thescan direction as the dimension perpendicular to the scan direction ofexposure fields 15 of purely refractive projection objectives.

FIG. 4 shows the exposure field 85 in the field plane of a projectionobjective for microlithography applications which is used as a stepperand has a square-shaped field 85 as exposure field 85. The elements inFIG. 4 which are analogous to those in FIG. 1 have the same referencenumerals raised by 70. In contrast to a scanner, a stepper functions insuch a way that the mask structure for the semiconductor element to beproduced, which is located in the object- or mask plane of theprojection objective, is projected in its entirety, i.e. without ascanning process, into the exposure field 85 in the field plane.However, this involves the projection objective providing largerexposure fields 85 than in the case of scanners. As an alternative forthe large exposure fields 85 in the case of steppers, the semiconductorelement can be exposed sequentially in a stepper in individual portions,using a so-called stitching technique. In this case, it is also possibleto use smaller exposure fields 85 than in the case of scanners. Theexposure field 85 in steppers can arbitrarily be made larger and smallerin the X-direction as well as the Y-direction by the REMA blades in theillumination system. The intensity distribution over the exposure field85 in steppers is completely homogeneous, so that the resultantdistribution has a top-hat- or rectangular profile in the X-direction aswell as in the Y-direction. To ensure that the steppers can be comparedto the scanners within the scope of this patent application, borderareas 77 and 79, located to the right and left at the borders of thestepper field perpendicular to the Y-direction. Furthermore, analogousto the scanners described herein, the intensity distribution in theX-direction is averaged over the Y-direction, which results in a top-hatdistribution of the kind shown in the lower part of FIG. 1, with thesame intensity value of 100% of the useful light for the central areas 5and 75, respectively, as for the border areas 7 and 77, respectively. Tomaintain the comparability with scanners, the stray light component ofsteppers is likewise defined as being averaged along the Y-direction.

FIG. 5 presents a schematic illustration of a projection objective 103and also a substitute model of a projection objective as a homogeneousglass cylinder 111 serving to explain the natural stray lightdistribution which occurs as a result in the field plane 105. In theupper part of FIG. 5, a schematic representation of a projectionobjective 103 is indicated by four lenses 109 along an optical axis 113.This projection objective 103 has the function of projecting an image ofa mask 101 which is located in a mask plane into a field plane 105. Themask to be projected is homogeneously illuminated for this purpose bylight 107 from an illumination system which is not shown in the drawing.The illumination system is capable of changing the angular distributionof the incident light rays 107 falling homogeneously on the mask 101,without thereby changing the intensity distribution over the mask. Thismakes it possible to have different so-called settings available for thesemiconductor manufacturer, which can be described in terms of thetheory of partially coherent images and which have the purpose thatcertain structures on the mask 101 can be projected into the smallestpossible image size.

The lower part of FIG. 5 represents, as a substitute model for theprojection objective 103, a homogeneous glass cylinder 111 which ishomogeneously illuminated by the light rays 107 which fall homogeneouslyon the mask 101. A glass cylinder 111 of this kind, which ishomogeneously illuminated over its cross-sectional area, will generateequal amounts of stray light within equal-sized surface elements of thecross-sectional area. If the glass cylinder 111 from the mask 101 to thefield plane 105 along the optical axis 113 is looked at as a series ofmany such homogeneously illuminated cross-sectional areas wherein theoverall intensity of the illumination decreases along the optical axis113 from the mask 101 to the field plane 105 due to absorption andscattering, one obtains a stray light component in the field plane 105,averaged over the scan direction Y, which conforms to the diagram at thelower right of FIG. 5. Due to the fact that each of the equal-sizedsurface elements of each cross-sectional area generates an equal amountof stray light, the proportion of stray light is higher in the centralarea 115 of the exposure field of the field plane 105 than in the borderarea 117 of the exposure field (as illustrated in the diagram at thelower right of FIG. 5), because the central area 115 receives the straylight of more mutually adjacent surface elements of each cross-sectionalarea than does the border area 117. This profile of the stray lightcomponent over the exposure field as illustrated in the lower right-handpart of FIG. 5, which results from the homogeneous illumination of acylindrical glass body, will be referred to hereinafter as the naturalprofile of the stray light component.

FIG. 6 shows the image-forming light ray pattern of a projectionobjective according to the principles of geometric optics to illustratethe concepts of field and pupil. The projection objective 123 in FIG. 6is shown as a so-called 4f system consisting in this schematicallysimplified representation of two lenses 129, between the latter a pupilplane 133, and two field-proximate planes 135, 137 in which the lenses129 are located. The projection objective projects an image of the mask121, which is homogeneously illuminated by the light rays 127, along theoptical axis 131 into the field plane 125. To explain theimage-projecting light ray pattern, three specific ray paths are shownfor the axis point of the mask 121, i.e., the principal ray 139 alongthe optical axis 131, the upper aperture ray or coma ray 141, and thelower aperture ray or coma ray 143. These aperture rays or coma rays arethose rays which leave the axis point at the maximum possible angle atwhich they can still be projected into an image by the projectionobjective. Also shown is the path of the principal ray 149 for theoutermost field point to be projected by the projection objective. Thepupil is defined as the area at whose center the principal rays 139, 149of all field points intersect each other and whose size is determined bythe aperture rays 141, 143. Thus, the pupil does not necessarily alwayshave to be in a pupil plane 133 as shown in FIG. 6, but a representationlike the one in FIG. 6 facilitates the explanation of the opticalconcepts of field and pupil. The pupil plane 133 according to FIG. 6 istherefore the location relative to the light propagation direction orZ-direction where the principal rays 139, 149 of the field points meeteach other. Since a principal ray 139 coincides with the optical axis,the pupil in FIG. 6 also is the location where all principal rays 139,149 of the field points intersect the optical axis. The principal rays139, 149 of the field points thus have no height, or distance from theoptical axis, in the pupil. The aperture rays 141, 143, on the otherhand, define the border of the pupil and thus have the maximum height,or maximum distance from the optical axis, of all possible rays in thepupil. The height, or distance from the optical axis, of the rays thusrepresents a suitable criterion as to whether an optical element in anobjective can be referred to as being near a pupil (pupil-proximate) ornear a field (field-proximate). If the height or distance of an apertureray 141, 143 of the axis point, or central field point, at a surface ofan optical element is more than six times the height of the principalray of the outermost projectable field point on the same surface, thenthe optical element will be referred to herein as being near a pupil (orpupil-proximate), otherwise it will be referred to herein as being neara field (or field-proximate), wherein in so-called RCR designs(refractive-catadioptric-refractive designs) the reference for thedistance of the rays in the elements of the Schupmann group G20 (seeFIG. 16) is their optical axis. Based on this criterion, it is clearthat the two lenses 129 in FIG. 6 are located, respectively, infield-proximate planes 135 and 137. Furthermore, field and pupil arerelated to each other through a spatial Fourier transform wherein theheight, or distance from the optical axis 131, of an image-forming ray139, 141, 143, 149 in the field corresponds to the angle between theimage-forming ray 139, 141, 143, 149 and the optical axis in the pupil.At the same time, the inverse relationship also holds, i.e., the anglebetween the image-forming ray 139, 141, 143, 149 and the optical axis inthe field corresponds to the height, or distance from the optical axis131, of the image-forming ray 139, 141, 143, 149 in the field. In otherwords, the path of the principal ray 149 of the outermost field pointthat can be projected has its maximum height, or greatest distance fromthe optical axis 131, in the image plane of the mask 121, with an angleof zero relative to the optical axis 131. The same ray path 149 crossesthe optical axis 131 at the center of the pupil plane 133 with themaximum angle of intersection, i.e. the height of the ray from theoptical axis 131 is minimal at this point, while the angle relative tothe optical axis 131 is maximal. Conversely, the aperture rays havetheir smallest heights and largest angles relative to the optical axis131 in the image plane of the mask 121 and the field plane 125, whiletheir greatest heights and smallest angles relative to the optical axis131 occur in the pupil plane 133. Based on this special relationshipbetween field and pupil, it is possible to perform interventions intothe light distribution in the pupil which have a uniform effect on everyfield point of the field. The simplest possibility is for example toconstrict the pupil with an aperture stop, so that all field points arelacking rays whose angle in the field is larger than the maximumpossible aperture angle allowed by the constricted pupil.

By an illumination system, the light rays 127 which are fallinghomogenously on the mask 121 are adapted in regard to their angulardistribution relative to the optical axis in order to meet customer'sdesired properties that specify so-called illumination settings, so thatdifferent areas with different intensities are formed in the pupil ofthe projection objective, whereby lenses near a pupil of the projectionobjective are illuminated differently depending on the illuminationsetting. For example, an annular setting in combination with a suitablemask structure has the consequence that lenses near a pupil arereceiving light only in border areas of the optically usable part of thelens. For an explanation of the working principle of the illuminationsettings in combination with the mask structures, the reader is referredto the pertinent literature concerning the theory of partially coherentimages of objects that are not self-luminous.

In the relationship between pupil, specifically lenses near a pupil, andstray light it is important that due to the three causes of Rayleighscattering, Mie scattering and geometric scattering, the elasticscattering of light of the wavelength λ which occurs at theinhomogeneities of the glass material always produces an angulardistribution that is symmetric around the direction of the useful lightray. This means that for field points at the border of the field, whoseprincipal rays are strongly angled in the pupil, and for a conventionalsetting with a small sigma value (which is a setting in which only thecentral area of the pupil, i.e. the area traversed by the principalrays, is being used), the resultant angular distributions of the straylight in pupil-proximate lenses are oriented outwards to the housing ofthe objective and away from the optical axis, so that on the way fromthe pupil to the field, stray light is absorbed by the housing of theobjective and by the lens mounts. The result of this is a stray lightcomponent profile over the field which, due to the stray lightabsorption, has a lower value in the border area 147 of the exposurefield than in the rest of the exposure field. For an annular setting onthe other hand, which uses the border area of the pupil and thus thearea traversed by the aperture rays, there is overall only aninsignificant difference in the angles of inclination of the aperturerays between field points of the border area and field points of thecentral area, but due to the proximity of the border area of the pupilto the housing of the objective, the part of the stray light that isscattered in the pupil under a large angle is absorbed most strongly.Since large angles in the pupil translate according to the Fouriertransform into large heights in the field, the stray light that isscattered in the pupil under a large angle is subject to absorption inthe housing of the objective and therefore lacking in the border area147 in comparison to the central area 145 of the exposure field.Accordingly, an annular illumination setting in particular (i.e. asetting where the light rays 127 fall on the mask 121 with rotationalsymmetry at angles of incidence within a narrowly defined angular range)does not lead to a profile of the stray light component that isqualitatively different from the profile obtained with a conventionalsetting. Consequently, that part of the variation of the stray lightcomponent averaged in the scan direction which occurs as a result ofdifferent settings can overall be considered negligible in relation tothe amount by which the stray light component, averaged in the scandirection, according to the measurement rule used herein varies over thefield.

In projection objectives for immersion lithography, the last lens withits strongly positive refractive power has the result that the pathlengths in the optical material are different for different fieldpoints. The relative path length difference of all image-forming rays ofa field point in the border area of the exposure field in comparison toall image-forming rays of the central field point of the exposure fieldfor such a lens alone can amount to a few percent. Consequently, sincethe stray light component due to inhomogeneities in the glass materialdepends directly on the path length traveled in the glass material bythe useful light, this leads particularly in strongly scatteringmaterial to a resultant stray light component profile over the fieldwith a lower value in the border area 147 of the exposure field than inthe central area 145.

In the context of FIGS. 5 and 6, a total of three different effects havebeen discussed, all of which lead to a stray light component, averagedover the scan direction, wherein the profile over the exposure field hasa stronger stray light component in the central area 145 than in theborder area 147 of the exposure field, as illustrated in the right-handpart of FIG. 6. All of these three effects result from the primary straylight due to elastic scattering of light at inhomogeneities in the glassmaterial and are, respectively, the natural stray light profile of ahomogenously illuminated glass body, the stray light profile of thelenses near a pupil, and the stray light profile due to the differencesin path length in strongly positive field lenses.

In addition to the effects just mentioned, which are due to the primarycause of stray light, i.e. the elastic scattering of light atinhomogeneities in the glass material, there is the superimposed straylight which is due to the scattering of light at surface irregularitieswhich, as mentioned above, represents a second primary cause of straylight. The lenses are usually polished to a uniform finish quality onall parts of the surface and consequently, the above train of reasoningthat the image-forming ray paths of field points from the border area ofthe field are overall more strongly inclined relative to the opticalaxis and relative to the refractive surfaces than the image-forming raypaths of field points from the central area, in combination with thefact that the angular distribution of the stray light is rotationallysymmetric to the direction of the useful light also in the case ofsurface scattering, leads to the conclusion that the scattering at thesurface irregularities likewise results in an average stray lightcomponent over the scan direction which is stronger in the central areaof the field than in the border area of the field and is characterizedby a profile over the field.

FIG. 7 shows a typical stray light component 151, averaged over the scandirection, for a microlithography projection objective of a currentdesign as a profile graph along the X-direction over the exposure fieldin accordance with the measurement rule observed herein. As is evident,the stray light component 151 is higher in the central area 155 of thefield with a value of 0.8% relative to the useful light than it is inthe border area 157 with a value of 0.6% relative to the useful light.

FIG. 8 schematically illustrates the optical part of a projectionexposure apparatus 201 for immersion lithography. The projectionexposure apparatus 201 has an excimer laser 203 as its light source witha wavelength of 193 nm. As an alternative, it is also possible to useother wavelengths such as 248 nm or 157 nm. An illumination system 205arranged in the light path downstream of the light source produces asharply delimited homogeneous illumination field in its image plane 207which is at the same time the object plane 207 of the projectionobjective 211 which follows in the light path. Normally in thisarrangement the ray geometry at the output side of the illuminationsystem 205 is adapted to the ray geometry at the input side of theprojection system 211. As mentioned above, the illumination system 205includes a mechanism for structuring the angular distribution of thelight rays 207 falling on the object plane 207 and for controlling thestate of polarization of the incident light rays. A so-called reticlestage holds the mask 213 in the object plane of the illumination systemand in accordance with the scanning process moves the mask along thescan direction 215. After the object plane 207 which at the same timerepresents the mask plane 207, the projection objective 211 follows nextin the light path, projecting a reduced image of the mask 213 onto awafer 219. The wafer 219 carries a light-sensitive so-called photoresist221 and is positioned so that the planar surface of the wafer 219 withthe photoresist 221 is located in the image plane 223, or field plane223, of the projection objective 211. The wafer 219 is held by aso-called wafer stage 217 and advanced at a rate that is synchronizedwith the movement of the mask 213. The wafer stage 217 also hasmanipulators which can move the wafer 219 along the optical axis 225 orperpendicular to it. Likewise incorporated in the wafer stage 217 is atilting manipulator which can tilt the wafer 219 about an axisperpendicular to the optical axis 225. The wafer stage 217 is designedspecifically for immersion lithography and includes a holder element 227with a shallow recess for the substrate 219 as well as a rim 229 tocontain the immersion liquid 231.

The projection objective 211 for immersion lithography applications hasan image-side numerical aperture NA that is larger than 1.0, preferablylarger than 1.2, and with even higher preference larger than 1.5. Theprojection objective 211 has as its last optical element before thefield plane 223 a planar-convex lens 233 whose underside 235 is the lastoptical surface of the projection objective 211 in the light path asseen in the direction of the light rays propagating from the mask planeto the field plane. This underside 235 is totally immersed in animmersion liquid 231.

The hemispherical planar-convex lens 233 consists preferably ofpolycrystalline material whose microscopic structure is illustrated inFIG. 9. Conceivably, further lenses 237 of a projection objective couldalso consist of polycrystalline material.

FIG. 9 shows the microscopic structure of a polycrystalline materialschematically and not true to scale. The material 300 shown here ispolycrystalline magnesium spinel (MgAl₂O₄) and has a large number ofdifferently oriented crystals 302 delimited by respective crystalboundaries 303. The mean crystal dimension in this example is around 25μm. Interspersed between the crystals 302 are hollow spaces, or bubbles304, whose mean dimension in this example is about 1 μm. Otherpolycrystalline materials are likewise conceivable for use as an opticalmaterial, for example other polycrystalline spinels, polycrystalline YAG[yttrium aluminum garnet (Y₃Al₅O₁₂)], polycrystalline LuAG [lutetiumaluminum garnet (Lu₃Al₅O₁₂)], polycrystalline magnesium oxide (MgO),polycrystalline beryllium oxide (BeO), polycrystalline aluminum oxide(Al₂O₃), polycrystalline yttrium oxide (Y₂O₃) or polycrystallinefluorides with a high refractive index, such as for example BaLiF₃ orLaF₃.

FIG. 10 shows the stray light component in percent relative to theuseful light of a homogeneous polycrystalline material of spinel with 40mm thickness as a function of the mean crystal dimension D according tothe corresponding stray light model presented in WO 2006/061225. Thisstray light model, besides taking the stray light L_(ret) into accountwhich results from the refractive index fluctuations due to thedifferent orientations of the crystals along a light path, also includesa stray light component I_(scat) which results from the total reflectiontaking place at the crystal boundaries 303. This adds up to a totalstray light component for the stray light, which is represented asI_(sum) in FIG. 10 and has its minimum for the crystal size marked bythe arrow P. Furthermore, a model-dependent stray light component of apolycrystalline material of spinel of 40 mm thickness is represented inFIG. 11, expressed in percent relative to the useful light as a functionof the mean bubble diameter according to the corresponding stray lightmodel in WO 2006/061255.

Based on the stray light models in WO 2006/061255, or in FIGS. 10 and11, only specific parameter ranges for the mean crystal size and themean bubble diameter in polycrystalline material are feasible for usingthis kind of material in projection objectives for microlithographyapplications, as the stray light component of the projection objectivewill otherwise become too large. However, FIGS. 10 and 11 lead to theconclusion that even if the parameter ranges that are optimal in regardto stray light are adhered to in the production of the polycrystallinespinel material, an optical element of spinel with a thickness of 40 mmwill still produce a stray light component of about 0.4% relative to theuseful light. By also considering the aforementioned natural stray lightdistribution of a body carrying a homogeneous flow of light, one arrivesat the result that for a last, field-proximate lens of polycrystallinematerial immediately before the field plane, the profile of the straylight component, averaged over the scan direction, has a variation overthe entire field plane of 0.4% relative to the useful light. The exactamount of variation over the exposure field in the field plane for thestray light component of such a field-proximate lens, averaged over thescan direction, depends on the exact geometry of the lens and theexposure field as well as on the distance of the lens from the fieldplane, and it is entirely possible for the variation to be only half aslarge as the aforementioned value. Insofar, a strongly positive singlelens of spinel, used as the last lens of the objective, has a variationof the stray light over the exposure field that is about half as largeas the variation of an entire projection objective of current design.

FIG. 12 represents a sketch to illustrate principal concepts regardingthe scattering at inhomogeneities 407 in the polycrystalline material ofa last lens 400 and regarding the concept of adapting the surfaceroughness 403 of a last lens, as well as the resultant stray lightdistributions 411, 413 over the field. In FIG. 12 a last lens 400 of aprojection objective is located before the field plane 405 which extendsperpendicular to the optical axis 401 immediately after the last lens400. The inhomogeneities of the glass material are symbolicallyindicated in the lens 400 as scatter lobes 407 which represent theangular distribution of the stray light. The stray light component 411of the lens 400 due to the inhomogeneities of the glass material (volumescatter), averaged over the scan direction and expressed as a percentagerelative to the useful light is shown in the mid-portion of FIG. 12 as aprofile graph over the field along the X-direction. Current Monte Carlosimulations concerning the stray light component 411 due to the volumescatter of a lens consisting of polycrystalline material and arranged inlast position before the field plane in the ray direction from the maskplane to the field plane lead to the result that the stray lightcomponent averaged over the scan direction and expressed as a percentageof the useful light is about 0.4% in the central area 415 of theexposure field and about 0.2% in the border area 417 of the exposurefield, thus confirming the stray light values of WO 2006/061225 whichhave been discussed above. To compensate for the stray light component411 due to the volume scatter of the last lens which consists of spinel,the surface roughness of the upper side 402, i.e. the side of the lastlens that faces away from the field plane 405, is increased in theborder zones 403, which produces the result of an added stray lightcomponent 413. The change of the surface roughness of the upper side 402is selected so that it results in an additional stray light component413 whose profile over the exposure field complements the stray lightcomponent 411 due to the volume scatter, so as to add up to an overallstray light component that is nearly constant. The added stray lightcomponent 413 due to the surface roughness, expressed as a percentage ofthe useful light and averaged over the scan direction, is shown in theright hand portion of FIG. 12 as a profile graph over the field alongthe X-direction. By changing the surface roughness on the upper side 402of the last lens, only a very small amount of additional stray light 413is introduced in the central area of the exposure field 415, in contrastto the border area 417 of the exposure field where the added amount ofstray light is about 0.5%, which compensates for the stray light 411which comes from the volume scatter of the last lens. The surfaceroughness of the upper side 402 does not necessarily have to be producedin a reworking operation; it can also be adapted in advance during theproduction process of the lens.

FIG. 13 shows the stray light component, expressed as a percentagerelative to the useful light, of a projection objective formicrolithography applications, which has been corrected in accordancewith the disclosure, averaged over the scan direction y and representedas a profile graph 501 in the X-direction along the field. The finelydotted line in FIG. 13 represents the stray light component, averagedover the scan direction, of a projection objective in which the lastlens element does not consist of polycrystalline material, in the formof a profile graph 503 along the X-direction over the exposure fieldwith a central area 505 and a border area 507. The variation over thefield is smaller than 0.2% for this stray light component, and thelatter is therefore considered a constant stray light component withinthe bounds of this application. The horizontal grid lines and the bands509 with a height of 0.2% serve as a graphic background to indicate therange within which a stray light component is considered constant withinthis application. The stray light component of a comparable projectionobjective in which the last lens consist of polycrystalline material isrepresented by a broken line with the reference symbol 502 in FIG. 13.The stray light component 502 exhibits a stronger variation over thefield than would be permissible for a constant stray light component509. A solid and heavier line 501 in FIG. 13 represents the stray lightcomponent of a projection objective that has been corrected inaccordance with the disclosure, with a last lens of polycrystallinematerial. This stray light component 501 of the projection objectivewhich has been corrected has a stray light component which in thecentral area 505 and in the border area 507 as well as in all fieldpoints in between amounts to about 1.3% relative to the useful light.Accordingly, this represents a very constant stray light component,averaged over the scan direction, with a variation over the exposurefield far below 0.2% relative to the useful light.

The disclosure is suited insofar not only for the correction ofprojection objectives with a last lens of polycrystalline material, butalso for the improvement of current projection objectives so that theywill have a constant stray light component with less than 0.2% variationover the exposure field.

FIG. 14 shows a so-called two-mirror design 2100 of a projectionobjective for immersion lithography with an image-side numericalaperture larger than 1. The design 2100 has been borrowed from FIG. 38of US 2005/0190435 A1, keeping the same reference symbols. Only thereference symbols for the areas 2003 of increased surface roughness arenewly added in comparison to FIG. 38 of US 2005/0190435 A1. The design2100 is drawn in FIG. 14 a in an X-Y sectional view and thus in a planethat is defined by the scan direction y and the direction of the opticalZ-axis, because the folded configuration of the ray path could not bevisualized otherwise. The same form of representation is also used inall of the catadioptric design discussed hereinafter. The mask plane2101 is projected by the first refractive objective group 2110 onto anextended intermediate image plane 2103. The first refractive group has apupil- or aperture plane A. The mirror group 2120 with the mirrors 2121and 2122 projects the extended intermediate image plane 2103 into afurther extended intermediate image plane 2104. The second refractiveobjective group 2130 projects the extended intermediate image plane 2104into the field plane 2102. The last lens before the field plane 2102 inthe direction of the light rays from the mask plane 2101 to the fieldplane 2102 carries the reference symbol 2150. The surface areas offield-proximate optical elements near the exposure field 2102 or nearthe intermediate field planes 2103 and 2104, which are suitable forcorrecting the variation of the stray light component over the exposurefield by increasing the surface roughness are indicated by a heaviersawtooth line 2003. For better clarity, the lower part of the secondrefractive group 2130 is shown in an enlarged view in FIG. 14 b. Furtherindicated by the shaded bars in FIG. 14 b is the area 2005 of thesurface of the last optical element 2150 before the field plane 2102 inthe direction of the light rays from the mask plane 2101 to the fieldplane 2102, where an aperture stop could be suitably arranged to reducestray light, in particular out-of-field stray light. This aperture stopcan be realized with mechanical field aperture stops between the lastoptical element 2150 and the field plane 2102. However, it is moreadvantageous to realize the aperture stop by masking the surface parts2005 of the last optical element which are indicated by the shaded barsin FIG. 14 b, because this creates no spatial interferences and has nodetrimental influence on the flow dynamics of the immersion liquid. Thismasking can be accomplished cost-effectively by placing an absorbent orreflective coating on the areas 2005 that are shaded in FIG. 14 b.

However, in the representation of the design in FIGS. 14 a and 14 b itshould be noted that the design is shown in a Y-Z sectional view andthus in the scanning direction, because the structural concept of thedesign could not be represented in an X-Z section, i.e. perpendicular tothe scanning direction. The heavier sawtooth lines 2003 in FIGS. 14 aand 14 b insofar indicate only the field-proximate surfaces which can beconsidered for an adaptation of the surface roughness, and on the otherhand only illustrate the principle that those areas 2003 of thefield-proximate surfaces which are met or traversed by rays of an outerfield point of the exposure field have a higher surface roughness. Theareas 2003 of the field-proximate surfaces with an increased surfaceroughness that are suitable for reducing the amount by which a straylight component, averaged over the scanning direction, variesperpendicular to the scanning direction over the exposure field can beillustrated better in an X-Z section of the design. Seen in an X-Zsectional view, the areas 2003 with the increased surface roughness arearranged on the optical elements in such a way that they are locatedequally at the borders to the right and left (relative to thex-direction) of the center of the optically used area, so that they havean equal effect on the stray light component, averaged over the scanningdirection, in the border areas to the right and left (relative to thex-direction) of the central area.

FIG. 15 shows a so-called four-mirror design PL1 of a projectionobjective for immersion lithography with an image-side numericalaperture of 1.2. The design PL1 has been borrowed from FIG. 9 of US2007/0024960 A1, keeping the same reference symbols. Only the referencesymbol for the field plane W1 is newly added in comparison to FIG. 9 ofUS 2007/0024960 A1. The mask plane R1 is projected onto an intermediateimage plane Q by the first catadioptric objective group G1 consisting ofthe purely refractive subgroup G11 with the lenses L1 to L4 and thecatadioptric subgroup consisting of the lens 5 and mirrors M1 and M2.The intermediate image plane Q is projected into the field plane W1immediately after the lens 18 by the second catadioptric objective groupG2 consisting of the two mirrors M3 and M4, the refractive subgroup G21with the lenses L6 and L7, the refractive subgroup G22 with the lensesL8 to L12, and the refractive subgroup G23 with the lenses L13 to L18. Apupil plane or aperture plane AS1 is located between the subgroups G22and G23. The broken lines extending the mirror surfaces M2 and M3illustrate the statement made above that catadioptric designs cannormally be described through the terminology of rotationally symmetricdesigns, even if the real ray path geometry or the real physical shapesof the optical elements of such a design no longer exhibit thisrotational symmetry. In order to retrace this thought process, thedesign PL1 shown in FIG. 15 has to be rotated about the optical axisAX1. After this rotation, all optical elements possess rotationalsymmetry relative to the optical axis AX1, and the optical axis AX1 isnow also the optical axis of all optical elements within the design PL1.

The field-proximate surface areas near the field plane W1, or near theintermediate image plane Q, in the direction of the light path from themask plane R1 to the field plane W1, which are suitable for correctingthe variation of the stray light component over the exposure field byincreasing the surface roughness are in this design PL1 all of themirror surfaces M1 to M4 and the surfaces of the lenses L5, L6 and L18.

FIG. 16 shows a so-called RCR design (refractive-catadioptric-refractivedesign) of a projection objective for immersion lithography with animage-side numerical aperture of 1.25. The design has been borrowed fromFIG. 19 of WO 2004/019128 A2, wherein the reference symbols have beenmaintained to the largest extent, except that each of the referencesymbols of the groups and lenses has been expanded with an added zero,while the reference symbol W1 for the field plane, the reference symbolM10 for the first direction-changing mirror, and the reference symbolM20 for the second direction-changing mirror have been newly added incomparison to FIG. 19 of WO 2004/019128 A2. The first refractiveobjective group G10 with the lenses L110 to L1100 projects the maskplane R1 into a first extended intermediate image area after the firstdirection-changing mirror M10. The catadioptric group G20 consisting ofthe lenses L210, L220 and a spherical mirror CM forms a so-calledSchupmann achromat for the correction of the longitudinal chromaticaberration and projects the first extended intermediate image area intoa second extended intermediate image area before the seconddirection-changing mirror M20. The second intermediate image plane isprojected into the field plane W1 immediately below the lens L3150 bythe second refractive objective group G30 with the lenses L310 to L3150.The second refractive objective group has a pupil plane or apertureplane identified as AS. As has already been mentioned above, the opticalaxis of the Schupmann achromat, or group G20, represents the referenceaxis for the definition of the concepts of field and pupil as usedherein in regard to all elements after the first direction-changingmirror M10 and before the second direction-changing mirror M20, becausein contrast to all other designs presented herein, the rotationalsymmetry of the design about the optical axis is broken by thesedirection-changing mirrors. The field-proximate surfaces near the fieldplane W1, or near the intermediate image plane Q, in the direction ofthe light path from the mask plane R1 to the field plane W1, which aresuitable for correcting the variation of the stray light component overthe exposure field by increasing the surface roughness are in this RCRdesign the direction-changing changing mirror surfaces M10 and M20 aswell as the surfaces of the lenses L100, L310 and L3150.

FIG. 17 shows a further two-mirror design 800 of a projection objectivefor immersion lithography with an image-side numerical aperture of 1.75.The design 800 has been borrowed from FIG. 8 of WO 2006/133801 A1,wherein the reference symbols have to the largest extent beenmaintained. Only the reference symbols of the objective groups G100 toG900 have been expanded in comparison to FIG. 8 of WO 2006/133801 A1 byadding double zeroes. The first refractive objective group ROP1 projectsthe mask plane OP into an extended intermediate image plane IMI1. Thefirst refractive group has a pupil plane or aperture plane identified asAS. The extended intermediate image plane IMI1 is projected into afurther extended intermediate image plane IMI2 by the mirror group COP2with the mirrors CM1 and CM2. The second refractive objective group ROP3projects the extended intermediate image plane IMI2 into the field planeIP. The last lens before the field plane IP in the direction of thelight rays from the mask plane OP to the field plane IP carries thereference symbol LOE and consists of two partial lenses LOE1 and LOE2with an immersion liquid IL between the partial lenses (see descriptionof FIG. 18).

The field-proximate surfaces near the field plane IP, or near theextended intermediate image planes IMI1 and IMI2, in the direction ofthe light path from the mask plane OP to the field plane IP, which aresuitable for correcting the variation of the stray light component overthe exposure field by increasing the surface roughness are in thisdesign 800 the mirror surfaces CM1 and CM2 as well as the surfaces ofthe lenses B800, LOE and the lens before CM1 in the direction of thelight rays from the mask plane OP to the image plane IP.

FIG. 18 shows as a detail of the design 800 of FIG. 17 the last lenselement LOE before the field plane IP in the direction of the light raysfrom the mask plane OP to the image plane IP. This lens element consistsof quartz glass for the partial lens LOE1 and sapphire for the partiallens LOE2, wherein the crystallographic axis in the latter is orientedin the direction CA parallel to the optical axis AX. Between the twopartial lenses LOE1 and LOE2 there is an immersion liquid. Othercrystalline materials with a high index of refraction are also mentionedin WO 2005/133801 A1 for the second partial lens LOE2, such as forexample spinel (MgAl₂O₄), YAG [yttrium aluminum garnet (Y₃Al₅O₁₂)],magnesium oxide (MgO), beryllium oxide (BeO), aluminum oxide (Al₂ 0 ₃),yttrium oxide (Y₂O₃) or lanthanum fluoride (LaF₃). In the context ofimmersion lithography, it is important to note the teaching of WO2006/133801 A1 that when a high image-side numerical aperture isspecified as a desired property in a design, the value of the image-sidenumerical aperture should not exceed the refractive index of the lastoptical element before the exposure field. It is insofar important fordesigns with a numerical aperture larger than 1.7, as in the case of thedesign 800, for the last lens element to have a refractive index largerthan 1.7 at the applicable operating wavelength. Sapphire, which is usedas the material of a second partial lens LOE2 in FIG. 18, has arefractive index of 1.92 at an operating wavelength of 193 nm and thushas according to the teachings of WO 2006/133801 A1 enough of anumerical distance from the image-side numerical aperture of 1.75 of thedesign 800. However, it would also not involve a major task to adapt thedesign 800 to a design in which the last lens before the exposure fieldconsists of polycrystalline material with a refractive index larger than1.7 at an operating wavelength of e.g. 193 nm and to simultaneouslyrealize high numerical aperture values around 1.7.

FIG. 19 shows a six-mirror design of a projection objective forapplications in so-called EUV (extreme ultraviolet) lithography. Thedesign has been borrowed from FIG. 1 of US 2004/0051857 A1, keeping to alarge extent the same reference symbols to which only the numeral 5 hasbeen added. The first catoptric objective group G15 projects the maskplane OB5 into the intermediate image IMI5 by the mirrors M15 and M25.The objective group includes the pupil plane or aperture plane APE5. Thesecond catoptric objective group G25 projects the intermediate imageIMI5 into the field plane IM5 by the mirrors M35, M45, M55, and M65.Projection objectives for EUV lithography normally consist of mirrors,as there are no materials in existence that are sufficiently transparentfor wavelengths below 100 nm. Insofar, the task of equalizing theprofile over the exposure field for the portion of the stray lightcomponent that results from inhomogeneities in the glass material doesnot present itself in these projection objectives. However, mirrors withthe same surface finish scatter the light about 16 times as strongly aslenses with a refractive index of about 1.5 in air. Consequently, EUVprojection objectives are much more critical than conventionalrefractive systems in regard to stray light that is due to the surfaceproperties of the optical elements. As an additional factor, not onlythe polish of the optical element itself but also the highly reflectivecoatings play a big part in EUV objectives as a source of stray light.Insofar, it is also of practical benefit in projection objectives usedfor EUV lithography to reduce the stray light component, averaged overthe scan direction, in its profile over the exposure field in accordancewith the disclosure, or to take measures to ensure in accordance withthis patent application that the stray light component, averaged overthe scan direction, has a constant profile over the exposure field. Thefield-proximate surfaces near the intermediate image plane IMI5, in thedirection of the light path from the mask plane OB5 to the field planeIM5, which are suitable for correcting the variation of the stray lightcomponent over the exposure field by increasing the surface roughnessare in this design the mirror surfaces M25, M35 and M45.

As the optically used areas on the mirrors of the projection objectiveare in many cases located at a considerable distance from the opticalaxis OA5 of the projection objective, the optical axis can no longerserve as reference axis for the distance under the definition that wasgiven above for distinguishing close-to-pupil and field-proximateelements in projection objectives for EUV lithography. Rather, thenormal vector at the geometric center point of an optically used area ofa surface is chosen to serve as new reference axis for the distanceaccording to which pupil-proximate and field-proximate elements inprojection objectives for EUV lithography are distinguished. If anaperture ray of the central field point of the exposure field on thesurface of an optical element has a distance from the thus definednormal vector that is six times as large as the distance that theprincipal ray of a border point of the exposure field on the samesurface of the optical element has from the normal vector, the opticalelement is referred to as pupil-proximate, otherwise it will be referredto as field-proximate.

As a possible example, FIG. 20 shows a distribution of the increase insurface roughness as an RMS value over the optically used area of theupper side of a last lens before the field plane in the direction of thelight rays from the mask plane to the field plane, which correlates tothe additional stray light component, averaged in the scan direction, inregard to its profile over the exposure field, with a smaller straylight component in the central area of the exposure field and a highstray light component in the border area of the exposure field, so thatas a result the stray light component, averaged over the scan direction,will have a smaller variation over the exposure field of the projectionobjective, or more specifically, that a stray light component of theprojection objective, averaged over the scan direction, is obtainedwhich is constant in the sense of this patent application. The scale onthe X-axis of the diagram is normalized so that the height of the borderof the optically used area in the positive X-direction on the upper sideof the last lens has a value of 1, and the height of the center of theoptically used area has a value of zero. The maximum amount for theincrease of the RMS value in this diagram of slightly more than 2 nm atthe left and right borders of the optically used area in comparison tothe RMS value at the center of the optically used area is sufficient atan operating wavelength of e.g. 193 nm in order to correct the variationof the stray light component of a projection objective, averaged overthe scan direction, which amounts to about 0.2% over the exposure field.This is based on the assumption of typical geometric relationships ofthe last lens, distances between the last lens and the exposure field,aspect ratios of the exposure field, as well as the refractive indicesof the last lens element according to the designs of FIGS. 14, 15, 16and 17. Depending on the different parameters, it is also possible thatdifferent values of about 0.1% to about 0.4% are obtained for the amountby which the stray light component, averaged over the scan direction,varies over the exposure field. If the amount by which the stray lightcomponent, averaged over the scan direction, varies over the exposurefield is to be corrected by more than 0.2%, the desired value for thesurface roughness is obtained by normalizing the diagram of FIG. 20accordingly. The profile of the surface roughness value in the diagramof FIG. 20 can be described by a function in the form of a root of ageneral polynomial, wherein the lateral distance from the centerrepresents the independent variable. This description has the advantagethat the coefficients obtained from it are advantageously suited for theprogramming of polishing machines such as for example polishing robots.However, the profiles that can be realized with the polishing machinesare not open to an arbitrary choice, as the polishing heads have afinite dimension which imposes limits on the curvatures of the curvesthat represent the profile of the surface roughness in the diagramsexemplified by FIG. 20. It is for example not possible for polishingmachines to realize the break at height 0 in the diagram curve of FIG.20, as the finite dimension of the polishing head will always have theconsequence that a surface roughness value different from zero willremain at the height 0. This would for example have the result of aresidual value of the additional stray light component 413 in thecentral area 415, as shown in FIG. 12.

FIG. 21 schematically illustrates the different methods whereby it ispossible to provide a projection objective for applications in the fieldof microlithography with an additional stray light component, averagedover the scan direction, whose profile over the exposure field is suchthat the stray light component of the projection objective, averaged inthe scan direction, has a reduced variation over the exposure field or,more specifically, that a stray light component of a projectionobjective, averaged in the scan direction, is obtained which is constantin the sense of this patent application. In a first step A, the straylight component of the projection objective is either simulated ordetermined from data of the components or data of the respective blanksAs an alternative first step B, it is possible to take measurements onthe projection objective itself or on a projection objective ofidentical design and thereby determine the variation of the stray lightcomponent over the exposure field of the projection objective. In asecond step, the surface roughness of a surface of a field-proximateoptical element or the surface roughness properties of several surfacesof a plurality of field-proximate optical elements are eitherappropriately adapted in advance during production, prior toinstallation in the projection objective, or subsequently altered by theappropriate amount, so that the stray light component, averaged in thescan direction, has a reduced variation over the exposure field or, morespecifically, that a stray light component of the projection objective,averaged in the scan direction, is obtained which in the sense of thispatent application is constant over the exposure field. The success ofthe measures taken in the second step is verified in a third step by ameasurement which is taken as part of a qualifying examination of theprojection objective. Depending on the result of the third step, theprojection objective is either accepted as having a sufficiently goodcorrection, or the process loops back to the second step, wherein thesurface roughness of the surface of the field-proximate element or ofthe surfaces of the field-proximate elements is changed from itsprevious value. These process steps two and three are repeated until thecorrection is found to be sufficient.

As an alternative to the foregoing method, it can be reasonable forprojection objectives in which one individual lens contributes a majorportion of the stray light component, to determine only the contributionof the individual lens in a first step of the method and to compensatethe contribution in a second step by an advance adaptation or subsequentalteration of the surface roughness, so that the qualification test ofthe projection objective can be performed in a third step. Under thisalternative procedure, the measurements can be performed on the lensitself in a first process step B, or the contribution of the lens isdetermined from measurements taken in a first process step B on a lensof the same design. As an alternative, the individual lens can besimulated as part of a first process step A, or the contribution fromthis lens can be determined from data that are obtained from the blankof the lens.

FIG. 22 schematically illustrates the process steps for producingmicrostructures on a wafer by using a projection exposure apparatus witha projection objective according to this patent application. In a firststep, a thin metal film is vapor-deposited on the wafer. Next, in asecond step, the wafer with the metal film is overlaid with aphotosensitive coating, the so-called photoresist. In a third step, theprojection exposure apparatus with a projection objective according tothe present patent application transfers the structures of a mask in themask plane in a scanning process to the currently addressed surface of asemiconductor element on the wafer by photographic exposure of thephotoresist. This step is repeated until all surfaces of allsemiconductor elements on the wafer have been exposed. Subsequently, thewafer with the exposed photoresist is developed, whereby the photoresistis removed from the wafer at those locations on the wafer that receiveda sufficient exposure. This makes it possible to remove the metal filmat the locations where the photoresist was removed in the precedingprocess step. This process step is called etching. In a next step, thewafer is ready for further treatment for which the wafer returns to thestarting point of the process of FIG. 22 or is directed to the startingpoint of another process in another apparatus.

Even though the disclosure has been described through the presentationof specific embodiments, those skilled in the pertinent art willrecognize numerous possibilities for variations and alternativeembodiments, for example by combining and/or exchanging features ofindividual embodiments. Accordingly, it will be understood by thoseskilled in the pertinent art that such variations and alternativeembodiments are considered as being included in the present disclosureand that the scope of the disclosure is limited only by the attachedpatent claims and their equivalents.

1. A projection objective configured to project an image in an objectplane into an exposure field in a field plane, the projection objectivecomprising: a plurality of optical elements including at least oneoptical element of polycrystalline material, wherein: the exposure fieldextends along a scan direction in the field plane; during use of theprojection objective, light in the exposure filed includes useful lightand a stray light component; the stray light component in the exposurefield, averaged over the scan direction, varies over the exposure fieldby less than 0.5% relative to the useful light; and the projectionobjective is configured to be used in microlithography.
 2. Theprojection objective according to claim 1, wherein: the plurality ofoptical elements are along a light ray path from the object plane to thefield plane; the optical element of polycrystalline material is a lastoptical element before the field plane in a ray direction of the lightray path from the object plane to the field plane; the polycrystallinematerial comprises a material selected from the group consisting of afluoride, an oxide of group II, an oxide of group III, an oxide of therare earths, garnet and spinel; and the optical element ofpolycrystalline material has a stray light component which varies overthe exposure field by more than 0.2% relative to the useful light in theexposure field.
 3. The projection objective according to claim 1,wherein: a maximum of the stray light component at the exposure field,averaged over the scan direction, is less than 2% relative to the usefullight; and a maximum of the stray light component outside the exposurefield, averaged over the scan direction, is less than 2% relative to theuseful light.
 4. The projection objective according to claim 1, whereinat least one surface of at least one field-proximate optical element hasa surface roughness configured to generate a stray light component thatcomplements the stray light component of the rest of the projectionobjective so that in the exposure field the stray light component of theprojection objective, averaged over the scan direction, varies over theexposure field by less than 0.5% relative to the useful light.
 5. Theprojection objective according to claim 4, wherein the at least onesurface includes an optically used area with a center and a border, thesurface roughness of the at least one surface increases from the centerof the optically used area to the border of the optically used area, andthe profile of the surface roughness of the at least one surface as afunction of a lateral distance from the center of the optically usedarea corresponds to a root function of a general polynomial function inwhich a lateral distance represents a variable quantity.
 6. Theprojection objective according to claim 5, wherein a difference insurface roughness of the at least one surface from the border of theoptically used area to the center of the optically used are is largerthan 0.5 nm RMS.
 7. The projection objective according to claim 4,wherein the surface roughness of the at least one surface has awavelength range of local undulation between 10 mm and 10 μm.
 8. Theprojection objective according to claim 1, wherein: the plurality ofoptical elements are along a light ray path from the object plane to thefield plane, including a last optical element before the field plane ina ray direction of the light ray path from the object plane to the fieldplane; a field aperture stop is between the last optical element and thefield plane; an optically used area extends in a plane of the fieldaperture stop; and the field aperture stop has an allowance for alateral dimension of less than 1 mm added to the optically used area inthe plane of the field aperture stop.
 9. The projection objectiveaccording to claim 4, wherein: the plurality of optical elements arealong a light ray path from the object plane to the field plane,including a last optical element before the field plane in a raydirection of the light ray path from the object plane to the fieldplane; the last optical element has an upper side and an underside;relative to the light ray direction from the object plane to the fieldplane, the upper side is before the underside; the underside is beforethe field plane in the light ray direction from the object plane to thefield plane; and the surface is the upper side of the last opticalelement.
 10. The projection objective according to claim 1, wherein: theplurality of optical elements are along a light ray path from the maskplane to the field plane, including a last optical element before thefield plane in a ray direction of the light ray path from the objectplane to the field plane; the last optical element has an upper side andan underside; relative to the light ray direction from the object planeto the field plane, the upper side is before the underside; theunderside is before the field plane in the light ray direction from theobject plane to the field plane; a field aperture stop is formed ofmasked off parts of the underside of the last optical element; themasking-off comprises a coating of an absorbent or reflective layer; anoptically used area extends on the underside of the last opticalelement; and the masking-off includes an allowance for a lateraldimension of less than 0.5 mm added to the optically used area.
 11. Aprojection objective configured to project an image in an object planeinto an exposure field of a field plane, wherein during use of theprojection objective, an additional stray light component with anon-constant profile over the exposure field is present in the fieldplane, and wherein the projection is configured to be used inmicrolithography.
 12. A projection objective configured to project animage in an object plane into an exposure field of a field plane, theprojection objective comprising: a mechanism configured to introduceinto the exposure field in the field plane an additional stray lightcomponent with a non-constant profile over the exposure field, whereinthe projection objective is configured to be used in microlithography.13. The projection objective according to claim 12, wherein the exposurefield in the field plane includes a central area and a border area, andthe additional stray light component is lower in the central area of theexposure field than in the border area of the exposure field.
 14. Theprojection objective according to claim 12, comprising a plurality ofoptical elements arranged along a light ray path from the object planeto the field plane, including at least one field-proximate opticalelement before the field plane in a ray direction along the light raypath from the object plane to the field plane, or which in the raydirection from the object plane to the field plane is arrangedimmediately before or after an intermediate object plane that isconjugate to the field plane, wherein at least one surface of the atleast one optical element has a surface roughness which produces theadditional stray light component with the non-constant profile over theexposure field, the surface includes an optically used area with acenter and a border, and the surface roughness of the surface increasesfrom the center of the optically used area to the border of theoptically used area.
 15. The projection objective according to claim 14,wherein: a difference in surface roughness from the border of theoptically used area to the center of the optically used area is largerthan 0.5 nm RMS; the surface roughness as a function of a lateraldistance from the center corresponds to a root function of a generalpolynomial function in which a lateral distance represents a variablequantity; and the surface roughness has a wavelength range of localundulation of between 10 mm and 10 μm.
 16. The projection objectiveaccording to claim 12, comprising a plurality of optical elementsarranged along a light ray path from the object plane to the fieldplane, including a last optical element before the field plane in a raydirection along the ray path from the object plane to the field plane isarranged, Wherein a field aperture stop is between the last opticalelement and the field plane, an optically used area extends in a planeof the field aperture stop, and the field aperture stop has an addedallowance for a lateral dimension of less than 1 mm.
 17. The projectionobjective according to claim 14, comprising a plurality of opticalelements arranged along a light ray path from the mask plane to thefield plane, including a last optical element before the field plane ina ray direction along the light ray path from the mask plane to thefield plane is arranged, wherein: the last optical element has an upperside and an underside; relative to the light ray direction from theobject plane to the field plane, the upper side is before the underside;the underside is before the field plane in the light ray direction fromthe object plane to the field plane; and the surface is the upper sideof the last optical element.
 18. The projection objective according toclaim 17, wherein the field aperture stop is formed of masked off partsof the underside of the last optical element, the masking-off comprisesa coating with of absorbent or reflective layer, an optically used areaextends on the underside of the last optical element, and themasking-off includes an allowance for a lateral dimension of less than0.5 mm added to the optically used area.
 19. The projection objectiveaccording to claim 12, comprising a plurality of optical elementsincluding at least one optical element comprising a polycrystallinematerial selected from the group consisting of a fluoride, an oxide ofgroup II, an oxide of group III, an oxide of the rare earths, garnet orspinel, wherein the optical element of polycrystalline material has astray light component which varies over the field by more than 0.2%relative to useful light in the exposure field.
 20. A method ofintroducing an additional stray light component of a projectionobjective configured to be used in microlithography, the projectionobjective configured to project an image in an object plane into anexposure field of a field plane, the projection objective comprising atleast one field-proximate surface having a surface roughness, the methodcomprising: prior to introducing the additional stray light component,adapting or altering the at least one field-proximate surface to obtainthe additional stray light component of the projection objective so thatthe additional stray light component has a non-constant profile over theexposure field.
 21. The method according to claim 20, comprising:simulating or measuring the stray light component within the exposurefield of the entire projection objective; and based on the simulation ormeasurement, adapting or altering the surface roughness of the at leastone field-proximate surface to obtain the additional stray lightcomponent.
 22. The method according to claim 21, wherein the surfaceroughness of the at least one field-proximate surface is determined frommeasurements taken on a second projection objective that is equal indesign to the first projection objective, and the measurement results ofthe second projection objective are carried over to the first projectionobjective.
 23. A method of generating a stray light component of aprojection objective configured to be used in microlithographyaccording, the projection objective being configured to project an imagein an object plane into an exposure field of a field plane, and theprojection objective including at least one field-proximate surfacehaving a surface roughness, the method comprising: prior to generatingthe stray light component, adapting or alerting a surface roughness ofat least one field-proximate surface so that an exposure field in thefield plane receives the stray light component of the projectionobjective, wherein: the exposure field extends along a scan direction inthe field plane; and the stray light component, averaged over the scandirection, varies over the exposure field by less than 0.5% relative tothe useful light.
 24. The method according to claim 23, comprising:simulating or measuring the stray light component within the exposurefield of the entire projection objective; and based on the simulation ormeasurement, adapting or altering the surface roughness of the at leastone field-proximate surface to obtain the stray light component.
 25. Themethod according to claim 24, wherein the required surface roughness ofthe at least one field-proximate surface is determined from measurementstaken on a second projection objective that is equal in design to thefirst projection objective, and wherein the measurement results of thesecond projection objective are carried over to the first projectionobjective.
 26. The method according to claim 23, comprising: simulatingor measuring the stray light component of at least one lens; and basedon the simulation or measurement, adapting or altering the surfaceroughness of the at least one field-proximate surface so that anexposure field in the field plane receives the stray light component ofthe projection objective.